diff --git a/CHANGELOG.md b/CHANGELOG.md
index c8094c424..a367b7ade 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 * Add new `SpecSuffixOnly` option to `RSpec/FilePath` cop. ([@zdennis][])
 * Allow `RSpec/RepeatedExampleGroupBody` to differ only by described_class. ([@robotdana][])
 * Fix `RSpec/FilePath` detection across sibling directories. ([@rolfschmidt][])
+* Improve the performance of `RSpec/SubjectStub` by an order of magnitude. ([@andrykonchin][])
 
 ## 1.39.0 (2020-05-01)
 
@@ -514,3 +515,4 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@zdennis]: https://github.com/zdennis
 [@robotdana]: https://github.com/robotdana
 [@rolfschmidt]: https://github.com/rolfschmidt
+[@andrykonchin]: https://github.com/andrykonchin
diff --git a/lib/rubocop/cop/rspec/subject_stub.rb b/lib/rubocop/cop/rspec/subject_stub.rb
index 3b60cf1b4..9c8b3f392 100644
--- a/lib/rubocop/cop/rspec/subject_stub.rb
+++ b/lib/rubocop/cop/rspec/subject_stub.rb
@@ -77,7 +77,7 @@ class SubjectStub < Cop
 
         def on_block(node)
           return unless example_group?(node)
-          return unless (processed_example_groups & node.ancestors).empty?
+          return if (processed_example_groups & node.ancestors).any?
 
           processed_example_groups << node
           @explicit_subjects = find_all_explicit_subjects(node)
@@ -94,7 +94,7 @@ def processed_example_groups
         end
 
         def find_all_explicit_subjects(node)
-          node.each_descendant(:block).each_with_object({}) do |child, h|
+          node.each_descendant(:block).with_object({}) do |child, h|
             name = subject(child)
             next unless name