diff --git a/changelog/fix_unknown_env_causing_cache_issues.md b/changelog/fix_unknown_env_causing_cache_issues.md
new file mode 100644
index 0000000000..f3e6469fa4
--- /dev/null
+++ b/changelog/fix_unknown_env_causing_cache_issues.md
@@ -0,0 +1 @@
+* [#1260](https://github.com/rubocop/rubocop-rails/issues/1260): Fix a performance regression caused by `Rails/UnknownEnv` when using Rails 7.1. ([@lukasfroehlich1][])
diff --git a/lib/rubocop/cop/rails/unknown_env.rb b/lib/rubocop/cop/rails/unknown_env.rb
index ebb62ce908..873c2c3177 100644
--- a/lib/rubocop/cop/rails/unknown_env.rb
+++ b/lib/rubocop/cop/rails/unknown_env.rb
@@ -87,7 +87,7 @@ def unknown_env_name?(name)
 
         def environments
           @environments ||= begin
-            environments = cop_config['Environments'] || []
+            environments = cop_config['Environments'].dup || []
             environments << 'local' if target_rails_version >= 7.1
             environments
           end
diff --git a/spec/rubocop/cop/rails/unknown_env_spec.rb b/spec/rubocop/cop/rails/unknown_env_spec.rb
index 1965000916..660ada1a44 100644
--- a/spec/rubocop/cop/rails/unknown_env_spec.rb
+++ b/spec/rubocop/cop/rails/unknown_env_spec.rb
@@ -92,6 +92,15 @@
       RUBY
     end
 
+    it 'does not mutate the cop config' do
+      expect_no_offenses(<<~RUBY)
+        Rails.env.local?
+        Rails.env == 'local'
+      RUBY
+
+      expect(cop_config['Environments'].include?('local')).to be(false)
+    end
+
     context 'when `Environments` is nil' do
       let(:cop_config) do
         {