diff --git a/Rakefile b/Rakefile
index 7c88acc..7b87b4f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -10,4 +10,9 @@ Rake::TestTask.new(:test) do |t|
   t.test_files = FileList["test/**/test_*.rb"]
 end
 
+desc "Run performance benchmarks, failing if any operation's runtime is scaling worse than expected"
+task :benchmark do
+  FileList["test/benchmark/*_benchmark.rb"].sort.each { |f| load f }
+end
+
 task default: %i[test standard]
diff --git a/lib/classlist.rb b/lib/classlist.rb
index 83e4841..a7f178c 100644
--- a/lib/classlist.rb
+++ b/lib/classlist.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "forwardable"
+require "set" # standard:disable Lint/RedundantRequireStatement -- needed on Ruby < 3.2
 
 require_relative "classlist/version"
 
@@ -46,9 +47,11 @@ def ==(other)
 
   # Adds the given tokens to the list, omitting any that are already present.
   def add(tokens)
-    entries = build_entries(tokens)
-    entries.each do |entry|
-      self.entries.push(entry) unless self.entries.include?(entry)
+    build_entries(tokens).each do |entry|
+      next if @entries_set.include?(entry)
+
+      entries.push(entry)
+      @entries_set << entry
     end
   end
 
@@ -57,12 +60,13 @@ def add_operation(other)
   end
 
   def include?(token)
-    entries.include?(token)
+    @entries_set.include?(token)
   end
   alias_method :contains, :include?
 
   def initialize(entries = [])
     @entries = build_entries(entries)
+    @entries_set = @entries.to_set
     @operations = []
   end
 
@@ -87,12 +91,20 @@ def merge(classlist)
   # Removes the specified tokens from the classlist, ignoring any that are not
   # present.
   def remove(tokens)
-    entries = build_entries(tokens)
-    entries.each do |entry|
-      self.entries.delete(entry)
+    build_entries(tokens).each do |entry|
+      next unless @entries_set.delete?(entry)
+
+      entries.delete(entry)
     end
   end
 
+  # Replaces all entries in the classlist with the given tokens.
+  def reset(tokens)
+    @entries = build_entries(tokens)
+    @entries_set = @entries.to_set
+    @entries
+  end
+
   # Replaces an existing token with a new token. If the first token doesn't
   # exist, #replace returns false immediately, without adding the new token to
   # the token list.
@@ -104,6 +116,8 @@ def replace(old_token, new_token)
     else
       index = entries.index(old_token)
       entries[index] = new_token
+      @entries_set.delete(old_token)
+      @entries_set << new_token
     end
 
     true
@@ -136,7 +150,7 @@ def to_s
   def toggle(token, force = nil)
     raise ArgumentError, "The token can not contain whitespace." if token.to_s.include?(" ")
 
-    if entries.include?(token)
+    if include?(token)
       remove(token) unless force == true
       result = false
     else
diff --git a/lib/classlist/reset.rb b/lib/classlist/reset.rb
index b6fcc2a..3ae65cb 100644
--- a/lib/classlist/reset.rb
+++ b/lib/classlist/reset.rb
@@ -6,12 +6,12 @@
 # classlist when merged.
 class Classlist::Reset < Classlist::Operation
   def merge(original)
-    original.entries.replace(entries)
+    original.reset(entries)
   end
 
   # #resolve changes the original classlist
   def resolve(original)
-    original.entries.replace(entries)
+    original.reset(entries)
 
     super
   end
diff --git a/test/benchmark/add_benchmark.rb b/test/benchmark/add_benchmark.rb
new file mode 100644
index 0000000..b8167fd
--- /dev/null
+++ b/test/benchmark/add_benchmark.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+require_relative "benchmark_helper"
+
+Bench.scale_check("#add with many distinct tokens", small: 500, large: 4000, expected_order: 1) do |n|
+  list = Classlist.new
+  list.add((0...n).map { |i| "c#{i}" })
+end
diff --git a/test/benchmark/benchmark_helper.rb b/test/benchmark/benchmark_helper.rb
new file mode 100644
index 0000000..3e1fb81
--- /dev/null
+++ b/test/benchmark/benchmark_helper.rb
@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+require "benchmark"
+
+$LOAD_PATH.unshift(File.expand_path("../../lib", __dir__))
+
+require "classlist"
+require "classlist/add"
+require "classlist/remove"
+require "classlist/reset"
+
+# Small harness for catching algorithmic regressions (O(n) -> O(n^2) and
+# worse), rather than measuring absolute speed. Absolute timings are too
+# noisy across machines/CI runners to assert on directly, but the *ratio* of
+# runtime between a small and a large input is a reasonably stable signal:
+# if it grows much faster than the algorithm's expected order, something
+# regressed.
+module Bench
+  class RegressionError < StandardError; end
+
+  # Times the given block once for `small` and once for `large`, and raises
+  # RegressionError unless the runtime scaled roughly as expected.
+  #
+  # expected_order: the Big-O exponent the operation should scale by, e.g.
+  #   1 for O(n), 2 for O(n^2).
+  # tolerance: how much slack to allow over the expected ratio before
+  #   flagging a regression, to absorb machine noise.
+  def self.scale_check(name, small:, large:, expected_order:, tolerance: 2)
+    small_time = Benchmark.realtime { yield(small) }
+    large_time = Benchmark.realtime { yield(large) }
+
+    size_ratio = large.to_f / small
+    max_allowed_ratio = (size_ratio**expected_order) * tolerance
+    actual_ratio = small_time.zero? ? 0 : large_time / small_time
+
+    ok = actual_ratio <= max_allowed_ratio
+
+    printf(
+      "%-42s n=%-5d %8.4fs  n=%-5d %8.4fs  ratio=%7.2f  max=%7.2f  %s\n",
+      name, small, small_time, large, large_time, actual_ratio, max_allowed_ratio,
+      ok ? "OK" : "FAIL"
+    )
+
+    return if ok
+
+    raise RegressionError,
+      "#{name}: time grew #{actual_ratio.round(2)}x for a #{size_ratio.round(2)}x increase in " \
+      "input size (expected at most #{max_allowed_ratio.round(2)}x for an O(n^#{expected_order}) " \
+      "operation with #{tolerance}x tolerance)"
+  end
+end
diff --git a/test/benchmark/incremental_add_and_render_benchmark.rb b/test/benchmark/incremental_add_and_render_benchmark.rb
new file mode 100644
index 0000000..491617d
--- /dev/null
+++ b/test/benchmark/incremental_add_and_render_benchmark.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require_relative "benchmark_helper"
+
+# Regression guard for https://github.com/substancelab/classlist/pull/19 -
+# in 1.1.1, Classlist#+ started mutating the receiver and pushing an
+# operation onto @operations that was never cleared. Every subsequent call
+# to #to_s/#to_a/#== re-resolved the *entire* history of operations, turning
+# this common "add a class, render, add another, render" pattern from
+# roughly O(n^2) into O(n^3).
+Bench.scale_check("incremental + and render on each step", small: 300, large: 2400, expected_order: 2) do |n|
+  list = Classlist.new(["base"])
+  n.times do |i|
+    list += Classlist.new(["c#{i}"])
+    list.to_s
+  end
+end
diff --git a/test/benchmark/merge_operations_benchmark.rb b/test/benchmark/merge_operations_benchmark.rb
new file mode 100644
index 0000000..7d9490b
--- /dev/null
+++ b/test/benchmark/merge_operations_benchmark.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require_relative "benchmark_helper"
+
+# Chaining a lot of + operations and only rendering once at the end - the
+# "build once, render once" pattern that should stay cheap even when the
+# incremental render-on-every-step pattern (see
+# incremental_add_and_render_benchmark.rb) does not.
+Bench.scale_check("chained + with a single render at the end", small: 300, large: 2400, expected_order: 1) do |n|
+  list = Classlist.new(["base"])
+  n.times { |i| list += Classlist.new(["c#{i}"]) }
+  list.to_s
+end
diff --git a/test/benchmark/remove_benchmark.rb b/test/benchmark/remove_benchmark.rb
new file mode 100644
index 0000000..279b4ca
--- /dev/null
+++ b/test/benchmark/remove_benchmark.rb
@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require_relative "benchmark_helper"
+
+# #remove is still O(n) per call (Array#delete has to shift elements), so
+# removing n tokens from an n-entry list is O(n^2) - unlike #add/#toggle,
+# which the entries Set made effectively O(1) per call.
+Bench.scale_check("#remove with many distinct tokens", small: 500, large: 4000, expected_order: 2) do |n|
+  tokens = (0...n).map { |i| "c#{i}" }
+  list = Classlist.new(tokens)
+  list.remove(tokens)
+end
diff --git a/test/benchmark/toggle_benchmark.rb b/test/benchmark/toggle_benchmark.rb
new file mode 100644
index 0000000..1c7f468
--- /dev/null
+++ b/test/benchmark/toggle_benchmark.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+require_relative "benchmark_helper"
+
+Bench.scale_check("#toggle with many distinct tokens", small: 500, large: 4000, expected_order: 1) do |n|
+  list = Classlist.new
+  n.times { |i| list.toggle("c#{i}") }
+end