Do not fork off extra processes if only a single file needs inspection

changelog/fix_speed_up_single_file_inspection.md

@@ -0,0 +1 @@
+* [#10903](https://github.com/rubocop/rubocop/pull/10903): Skip forking off extra processes for parallel inspection when only a single file needs to be inspected. ([@wjwh][])

lib/rubocop/runner.rb

@@ -64,6 +64,10 @@ def aborting?
     # instances that each inspects its allotted group of files.
     def warm_cache(target_files)
       saved_options = @options.dup
+      if target_files.length <= 1
+        puts 'Skipping parallel inspection: only a single file needs inspection' if @options[:debug]
+        return
+      end
       puts 'Running parallel inspection' if @options[:debug]
       %i[autocorrect safe_autocorrect].each { |opt| @options[opt] = false }
       Parallel.each(target_files) { |target_file| file_offenses(target_file) }

spec/rubocop/cli/options_spec.rb

@@ -14,6 +14,11 @@
   describe '--parallel' do
     if RuboCop::Platform.windows?
       context 'on Windows' do
+        before do
+          create_file('test_1.rb', ['puts "hello world"'])
+          create_file('test_2.rb', ['puts "what a lovely day"'])
+        end
+
         it 'prints a warning' do
           cli.run ['-P']
           expect($stderr.string).to include('Process.fork is not supported by this Ruby')
@@ -35,7 +40,9 @@
       context 'on Unix-like systems' do
         it 'prints a message if --debug is specified' do
           cli.run ['--parallel', '--debug']
-          expect($stdout.string).to match(/Running parallel inspection/)
+          expect($stdout.string).to match(
+            /Skipping parallel inspection: only a single file needs inspection/
+          )
         end
 
         it 'does not print a message if --debug is not specified' do