diff --git a/.rubocop.yml b/.rubocop.yml
index c96c7123c1679b835b717a01d32348c2d7a8e85b..ccceea45963969d06e4e964826e5576dbe8674eb 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -937,10 +937,9 @@ Lint/Void:
 
 ##################### Performance ############################
 
-# TODO: Enable Casecmp Cop.
 # Use `casecmp` rather than `downcase ==`.
 Performance/Casecmp:
-  Enabled: false
+  Enabled: true
 
 # TODO: Enable DoubleStartEndWith Cop.
 # Use `str.{start,end}_with?(x, ..., y, ...)` instead of
diff --git a/app/models/repository.rb b/app/models/repository.rb
index a4b42d7226d2e8543fafe537907c1de0775e6b05..de7e163078dd09f6210d96705c02e1fa14e4bbfb 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -453,7 +453,7 @@ class Repository
   def version
     cache.fetch(:version) do
       tree(:head).blobs.find do |file|
-        file.name.downcase == 'version'
+        file.name.casecmp('version').zero?
       end
     end
   end
diff --git a/lib/gitlab/database.rb b/lib/gitlab/database.rb
index 6f9da69983a4a227793d1ffdf97beba048793cbb..42bec913a45ce7b4ff1cf7edd4b815844d1bcbbf 100644
--- a/lib/gitlab/database.rb
+++ b/lib/gitlab/database.rb
@@ -5,11 +5,11 @@ module Gitlab
     end
 
     def self.mysql?
-      adapter_name.downcase == 'mysql2'
+      adapter_name.casecmp('mysql2').zero?
     end
 
     def self.postgresql?
-      adapter_name.downcase == 'postgresql'
+      adapter_name.casecmp('postgresql').zero?
     end
 
     def self.version
diff --git a/lib/gitlab/markup_helper.rb b/lib/gitlab/markup_helper.rb
index a5f767b134d2d65f5d20f819f2f7345fc1d1dfc9..dda371e65548c2d221ae983d0bd86cd7ab377ed6 100644
--- a/lib/gitlab/markup_helper.rb
+++ b/lib/gitlab/markup_helper.rb
@@ -40,7 +40,7 @@ module Gitlab
     # Returns boolean
     def plain?(filename)
       filename.downcase.end_with?('.txt') ||
-        filename.downcase == 'readme'
+        filename.casecmp('readme').zero?
     end
 
     def previewable?(filename)