diff --git a/.duel.rb b/.duel.rb
new file mode 100644
index 0000000..5c938d5
--- /dev/null
+++ b/.duel.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+# Duel's configuration: https://github.com/sferik/duel-ruby/blob/main/docs/configuration.md
+#
+# Duel follows conventions, so set only what differs: benchmarks are in
+# duel/**/*_duel.rb, each version's lib/ is first on the load path, a
+# change is compared when it touches lib/ or ext/, and releases are
+# tagged v*.
+Duel.configure do |c|
+  # The library each version's workers require (from the gemspec, unless
+  # this says otherwise).
+  # c.library = "multi_json"
+end
diff --git a/.github/workflows/perf.yml b/.github/workflows/perf.yml
new file mode 100644
index 0000000..22ffbe0
--- /dev/null
+++ b/.github/workflows/perf.yml
@@ -0,0 +1,64 @@
+name: Perf
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  schedule:
+    - cron: "0 4 * * *"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pull-requests: write # to comment on a pull request with the report
+
+jobs:
+  duel:
+    name: Duel
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0 # the baseline comes from history, tags included
+      # (And whatever else your tests need set up: packages, services.)
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.4"
+          bundler-cache: true
+      # Against the main branch for a pull request, what was there before
+      # for a push, and the last release on a schedule (slow creep). Duel
+      # adds its report to the job's summary, and writes it to duel.md for
+      # the comment.
+      - name: Compare
+        env:
+          DUEL_BASE: ${{ github.event_name == 'schedule' && 'last-release' || github.event_name == 'push' && github.event.before || '' }}
+          # (Run by hand, it compares anyway: with nothing changed, the code
+          # with itself, which checks the setup.)
+          DUEL_FORCE: ${{ github.event_name == 'workflow_dispatch' && '1' || '' }}
+        run: |
+          # (A branch's first push, or one that rewrote history, has no
+          # commit before it here to compare against.)
+          if [ "$GITHUB_EVENT_NAME" = push ] && ! git cat-file -e "$DUEL_BASE^{commit}" 2>/dev/null; then
+            echo "::notice::No commit before this push to compare against"
+            exit 0
+          fi
+          bundle exec duel --markdown "$RUNNER_TEMP/duel.md"
+      # The report, as a comment on the pull request: one comment, updated
+      # on every push. (A pull request from a fork gets no comment, as its
+      # token can't write one: the job's summary has the report.)
+      - name: Comment on the pull request
+        if: ${{ !cancelled() && github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository }}
+        env:
+          GH_TOKEN: ${{ github.token }}
+          PR: ${{ github.event.pull_request.number }}
+        run: |
+          [ -f "$RUNNER_TEMP/duel.md" ] || exit 0
+          marker="<!-- duel -->"
+          { echo "$marker"; cat "$RUNNER_TEMP/duel.md"; } > "$RUNNER_TEMP/comment.md"
+          ids=$(gh api --paginate "repos/$GITHUB_REPOSITORY/issues/$PR/comments" \
+            --jq ".[] | select(.user.login == \"github-actions[bot]\" and (.body | startswith(\"$marker\"))) | .id")
+          if [ -n "$ids" ]; then
+            gh api --method PATCH "repos/$GITHUB_REPOSITORY/issues/comments/${ids%%$'\n'*}" -F body=@"$RUNNER_TEMP/comment.md" > /dev/null
+          else
+            gh api --method POST "repos/$GITHUB_REPOSITORY/issues/$PR/comments" -F body=@"$RUNNER_TEMP/comment.md" > /dev/null
+          fi
diff --git a/.rubocop.yml b/.rubocop.yml
index 991328b..f1b424b 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -7,7 +7,15 @@ plugins:
  - rubocop-rake
  - standard-performance
 
+# Merge AllCops/Include with RuboCop's defaults, so the list below adds
+# to them. Dotfiles are left out unless they're named there.
+inherit_mode:
+  merge:
+    - Include
+
 AllCops:
+  Include:
+    - ".duel.rb"
   Exclude:
     - ".git/**/*"
     - "tmp/**/*"
diff --git a/Gemfile b/Gemfile
index a7233a3..ae18bc4 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,6 +6,8 @@ gem "activesupport", ">= 8", require: false
 gem "concurrent-ruby", ">= 1.2", require: false
 gem "json", ">= 2", require: false
 
+# Duel requires Ruby 3.4 or later
+gem "duel", ">= 0.1" if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.4")
 gem "minitest", ">= 5.20"
 gem "mutant-minitest", ">= 0.12"
 gem "rake", ">= 13.1"
@@ -30,3 +32,6 @@ gem "jrjackson", ">= 0.4.18", platforms: [:jruby], require: false
 gem "oj", ">= 3", platforms: %i[ruby windows], require: false
 
 gemspec
+
+# For profiles of regressions (StackProf runs on CRuby only)
+gem "stackprof", platforms: :mri
diff --git a/README.md b/README.md
index 596b3a2..711863b 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,7 @@
 [![Mutant](https://github.com/sferik/multi_json/actions/workflows/mutant.yml/badge.svg)][mutant]
 [![Typecheck](https://github.com/sferik/multi_json/actions/workflows/typecheck.yml/badge.svg)][typecheck]
 [![Docs](https://github.com/sferik/multi_json/actions/workflows/docs.yml/badge.svg)][docs]
+[![Duel](https://raw.githubusercontent.com/sferik/duel-ruby/main/assets/badge.svg)](https://github.com/sferik/duel-ruby)
 [![Maintainability](https://qlty.sh/badges/fde3f4a8-c331-44be-b1e6-45842137def9/maintainability.svg)][qlty]
 [![Gem Version](https://badge.fury.io/rb/multi_json.svg)][gem]
 
diff --git a/Rakefile b/Rakefile
index 6016818..ea8c287 100644
--- a/Rakefile
+++ b/Rakefile
@@ -55,3 +55,10 @@ desc "Run the default task"
 default_tasks = %i[test lint mutant yardstick]
 default_tasks << :steep if Rake::Task.task_defined?(:steep)
 task default: default_tasks
+
+begin
+  require "duel/rake_task"
+  Duel::RakeTask.new
+rescue LoadError
+  # Duel requires Ruby 3.4 or later
+end
diff --git a/duel/adapter_duel.rb b/duel/adapter_duel.rb
new file mode 100644
index 0000000..c48f542
--- /dev/null
+++ b/duel/adapter_duel.rb
@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+# What runs around every parse and generate: resolving an adapter by
+# name, a block with a temporary adapter, the options cache when it's
+# full, and the deprecated names that older callers still use.
+
+require_relative "duel_helper"
+
+duel "adapter: resolve one by name" do
+  MultiJSON.current_adapter(adapter: :oj)
+end
+
+duel "adapter: parse in with_adapter" do
+  MultiJSON.with_adapter(:oj) { MultiJSON.parse('{"a":1}') }
+end
+
+# A store of its own, so that its evictions leave the shared caches
+# alone. It holds the most entries it can, and every fetch is of the key
+# it evicted longest ago, so every fetch misses and evicts one.
+duel "options: a cache miss in a full cache" do
+  once do
+    @store = MultiJSON::OptionsCache::Store.new
+    @keys = Array.new(MultiJSON::OptionsCache.max_cache_size + 1) { |index| {max_nesting: index + 1}.freeze }
+    @value = {}.freeze
+    @next = 0
+    @keys.each { |key| @store.fetch(key) { @value } }
+  end
+  key = @keys[@next]
+  @next = (@next + 1) % @keys.size
+  @store.fetch(key) { @value }
+end
+
+duel "legacy: MultiJson.load" do
+  once { MultiJson.load("{}") } # warns once, before it's timed
+  MultiJson.load('{"a":1}')
+end
+
+duel "legacy: MultiJSON.dump" do
+  once do
+    MultiJSON.dump({}) # warns once, before it's timed
+    @object = DuelPayloads.flat_scalars(40)
+  end
+  MultiJSON.dump(@object)
+end
diff --git a/duel/duel_helper.rb b/duel/duel_helper.rb
new file mode 100644
index 0000000..f186d98
--- /dev/null
+++ b/duel/duel_helper.rb
@@ -0,0 +1,95 @@
+# frozen_string_literal: true
+
+require "json"
+
+# Payloads for the benchmarks, in the shapes and sizes of benchmark.rb's
+# matrix: flat objects of scalars, nested records, and arrays of numbers
+# and strings. Every one is built from its index alone, so both versions'
+# workers parse and generate exactly the same documents.
+module DuelPayloads
+  COUNTRIES = %w[US DE BR JP IN AU GB CA].freeze
+  EVENT_TYPES = %w[create update sync archive].freeze
+  RECORD_TYPES = %w[user team account project].freeze
+
+  module_function
+
+  # A Hash of count fields, cycling through integers, floats, strings,
+  # booleans, nulls and short arrays of strings
+  def flat_scalars(count)
+    Array.new(count) { |index| ["field_#{index}", scalar_value(index)] }.to_h
+  end
+
+  # A Hash of count user records, each with nested objects and arrays
+  def nested_records(count)
+    {
+      "meta" => {"count" => count, "generated_at" => "2026-01-01T00:00:00Z", "source" => "benchmark"},
+      "data" => Array.new(count) { |index| record(index) }
+    }
+  end
+
+  # A Hash of long arrays of integers, floats, booleans and strings, and
+  # a matrix of integers
+  def array_heavy(count)
+    {
+      "ints" => Array.new(count) { |index| index },
+      "floats" => Array.new(count) { |index| ((index * 7) % 100_000) / 31.0 },
+      "booleans" => Array.new(count, &:even?),
+      "strings" => Array.new(count / 2) { |index| token("str", index, 18) },
+      "matrix" => Array.new(count / 25) { |row| Array.new(20) { |column| (row * 20) + column } }
+    }
+  end
+
+  # The JSON text of an object, frozen, as an application receives it
+  def json(object) = JSON.generate(object).freeze
+
+  def scalar_value(index)
+    case index % 6
+    when 0 then index
+    when 1 then ((index * 13) % 10_000) / 10.0
+    when 2 then token("value", index, 20)
+    when 3 then index.even?
+    when 4 then nil
+    else [token("tag", index, 6), token("tag", index + 1, 6), token("tag", index + 2, 6)]
+    end
+  end
+
+  def record(index)
+    {
+      "id" => index,
+      "external_id" => token("user", index, 10),
+      "type" => RECORD_TYPES[index % RECORD_TYPES.length],
+      "active" => (index % 3).zero?,
+      "profile" => profile(index),
+      "metrics" => metrics(index),
+      "events" => Array.new(3) { |offset| event(index + offset) }
+    }
+  end
+
+  def profile(index)
+    {
+      "name" => token("name", index, 14),
+      "email" => "user#{index}@example.test",
+      "country" => COUNTRIES[index % COUNTRIES.length],
+      "segments" => Array.new(4) { |offset| token("segment", index + offset, 6) }
+    }
+  end
+
+  def metrics(index)
+    used = index % 500
+    {
+      "logins" => index * 3,
+      "score" => ((index * 17) % 10_000) / 100.0,
+      "quota" => {"used" => used, "limit" => 500, "overage" => used > 450}
+    }
+  end
+
+  def event(index)
+    {
+      "at" => format("2026-02-%<day>02dT12:%<minute>02d:00Z", day: (index % 28) + 1, minute: index % 60),
+      "type" => EVENT_TYPES[index % EVENT_TYPES.length],
+      "delta" => (index % 17) - 8
+    }
+  end
+
+  def token(prefix, index, width) = "#{prefix}_#{index.to_s(36).rjust(width, "0")}"
+end
diff --git a/duel/generate_duel.rb b/duel/generate_duel.rb
new file mode 100644
index 0000000..fbb1b2e
--- /dev/null
+++ b/duel/generate_duel.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+# MultiJSON.generate, through the adapter MultiJSON picks by default (the
+# JSON gem), across benchmark.rb's shapes and sizes, and through each
+# adapter's handling of options such as pretty.
+
+require_relative "duel_helper"
+
+duel "generate: a tiny hash" do
+  MultiJSON.generate({"a" => 1})
+end
+
+duel "generate: a small flat object" do
+  once { @object = DuelPayloads.flat_scalars(40) }
+  MultiJSON.generate(@object)
+end
+
+duel "generate: a small set of records" do
+  once { @object = DuelPayloads.nested_records(8) }
+  MultiJSON.generate(@object)
+end
+
+duel "generate: a large set of records" do
+  once { @object = DuelPayloads.nested_records(384) }
+  MultiJSON.generate(@object)
+end
+
+duel "generate: a large array-heavy document" do
+  once { @object = DuelPayloads.array_heavy(25_000) }
+  MultiJSON.generate(@object)
+end
+
+duel "generate: pretty" do
+  once { @object = DuelPayloads.nested_records(8) }
+  MultiJSON.generate(@object, pretty: true)
+end
+
+duel "generate: with Oj" do
+  MultiJSON.generate({"a" => 1}, adapter: :oj)
+end
+
+duel "generate: pretty, with Oj" do
+  MultiJSON.generate({"a" => 1}, adapter: :oj, pretty: true)
+end
+
+duel "generate: through fast_jsonparser" do
+  once { @object = DuelPayloads.nested_records(8) }
+  MultiJSON.generate(@object, adapter: :fast_jsonparser)
+end
diff --git a/duel/parse_duel.rb b/duel/parse_duel.rb
new file mode 100644
index 0000000..efd4a45
--- /dev/null
+++ b/duel/parse_duel.rb
@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+# MultiJSON.parse, through the adapter MultiJSON picks by default (the
+# JSON gem), across benchmark.rb's shapes and sizes, and through each
+# path that parse takes before and after the adapter: blank and scalar
+# input, options, IO input, binary input, and parse errors.
+
+require "stringio"
+require_relative "duel_helper"
+
+duel "parse: a tiny object" do
+  MultiJSON.parse('{"a":1}')
+end
+
+duel "parse: a scalar" do
+  MultiJSON.parse("42")
+end
+
+duel "parse: blank input" do
+  MultiJSON.parse("  \n")
+end
+
+duel "parse: a small flat object" do
+  once { @json = DuelPayloads.json(DuelPayloads.flat_scalars(40)) }
+  MultiJSON.parse(@json)
+end
+
+duel "parse: a small set of records" do
+  once { @json = DuelPayloads.json(DuelPayloads.nested_records(8)) }
+  MultiJSON.parse(@json)
+end
+
+duel "parse: a medium set of records" do
+  once { @json = DuelPayloads.json(DuelPayloads.nested_records(64)) }
+  MultiJSON.parse(@json)
+end
+
+duel "parse: a large set of records" do
+  once { @json = DuelPayloads.json(DuelPayloads.nested_records(384)) }
+  MultiJSON.parse(@json)
+end
+
+duel "parse: a large array-heavy document" do
+  once { @json = DuelPayloads.json(DuelPayloads.array_heavy(25_000)) }
+  MultiJSON.parse(@json)
+end
+
+duel "parse: with symbolize_names" do
+  once { @json = DuelPayloads.json(DuelPayloads.nested_records(8)) }
+  MultiJSON.parse(@json, symbolize_names: true)
+end
+
+duel "parse: from an IO" do |n|
+  once { @json = DuelPayloads.json(DuelPayloads.nested_records(8)) }
+  ios = untimed { Array.new(n) { StringIO.new(@json) } }
+  ios.each { |io| MultiJSON.parse(io) }
+end
+
+duel "parse: binary-encoded input" do
+  once { @json = DuelPayloads.json(DuelPayloads.nested_records(64)).b.freeze }
+  MultiJSON.parse(@json)
+end
+
+duel "parse: invalid JSON" do
+  MultiJSON.parse('{"a":[1,2,')
+rescue MultiJSON::ParseError
+  nil
+end
+
+duel "parse: with Oj" do
+  MultiJSON.parse('{"a":1}', adapter: :oj)
+end
+
+duel "parse: with fast_jsonparser" do
+  MultiJSON.parse('{"a":1}', adapter: :fast_jsonparser)
+end