diff --git a/CHANGELOG.md b/CHANGELOG.md
index 59728345..ed25cfaf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ### Unreleased
 
+* (patch) RSpec: Speed up the time tracker formatter by recording test path timings in place instead of rebuilding the whole collection after each top-level example group.
+
 ### 10.0.1
 
 * Add support for [File Paths Encryption](https://docs.knapsackpro.com/ruby/encryption/) to [Retry only Failures](https://docs.knapsackpro.com/ruby/retry-only-failures/).
diff --git a/lib/knapsack_pro/formatters/time_tracker.rb b/lib/knapsack_pro/formatters/time_tracker.rb
index 01baabd6..d277ddab 100644
--- a/lib/knapsack_pro/formatters/time_tracker.rb
+++ b/lib/knapsack_pro/formatters/time_tracker.rb
@@ -81,7 +81,7 @@ def example_group_finished(notification)
 
         add_hooks_time(@group, @time_all_by_group_id_path)
         @time_all_by_group_id_path = Hash.new(0)
-        @paths = merge(@paths, @group)
+        merge_into(@paths, @group)
         @group = {}
       end
 
@@ -198,6 +198,18 @@ def merge(h1, h2)
         end
       end
 
+      # `merge` rebuilds the whole accumulator, which is quadratic when called
+      # once per top level group, so accumulate in place instead.
+      def merge_into(accumulator, other)
+        other.each do |path, example|
+          if accumulator.key?(path)
+            accumulator[path][:time_execution] += example[:time_execution]
+          else
+            accumulator[path] = example
+          end
+        end
+      end
+
       def now
         KnapsackPro::Utils.time_now
       end