diff --git a/CHANGELOG.md b/CHANGELOG.md
index b8c592457..c4fd3b59e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+* Improve performance when user does not override default RSpec Pattern config. ([@walf443][])
+
 ## 1.20.1 (2017-11-15)
 
 * Add "without" to list of default allowed prefixes for `RSpec/ContextWording`. ([@bquorning][])
diff --git a/lib/rubocop/cop/rspec/cop.rb b/lib/rubocop/cop/rspec/cop.rb
index 325cc4b39..d81604f31 100644
--- a/lib/rubocop/cop/rspec/cop.rb
+++ b/lib/rubocop/cop/rspec/cop.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RuboCop
   module Cop # rubocop:disable Style/Documentation
     WorkaroundCop = Cop.dup
@@ -42,6 +44,11 @@ class Cop < WorkaroundCop
         DEFAULT_CONFIGURATION =
           RuboCop::RSpec::CONFIG.fetch('AllCops').fetch('RSpec')
 
+        DEFAULT_PATTERN_RE = Regexp.union(
+          DEFAULT_CONFIGURATION.fetch('Patterns')
+                               .map(&Regexp.public_method(:new))
+        )
+
         # Invoke the original inherited hook so our cops are recognized
         def self.inherited(subclass)
           RuboCop::Cop::Cop.inherited(subclass)
@@ -58,12 +65,25 @@ def relevant_rubocop_rspec_file?(file)
         end
 
         def rspec_pattern
-          Regexp.union(rspec_pattern_config.map(&Regexp.public_method(:new)))
+          if rspec_pattern_config?
+            Regexp.union(rspec_pattern_config.map(&Regexp.public_method(:new)))
+          else
+            DEFAULT_PATTERN_RE
+          end
         end
 
-        def rspec_pattern_config
+        def all_cops_config
           config
             .for_all_cops
+        end
+
+        def rspec_pattern_config?
+          return unless all_cops_config.key?('RSpec')
+          all_cops_config.fetch('RSpec').key?('Patterns')
+        end
+
+        def rspec_pattern_config
+          all_cops_config
             .fetch('RSpec', DEFAULT_CONFIGURATION)
             .fetch('Patterns')
         end