diff --git a/CHANGELOG.md b/CHANGELOG.md
index b9b2cdf..52e1cbe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,8 @@
 ## Ruby CSS Parser CHANGELOG
 
 ### Unreleased
+* Speed up `RuleSet::Declarations#to_s` ~20x: its clean-up regexp matched the empty string at every character (`[\n\r\f\t]*`), making `gsub!` replace at every position; the unreachable `\s+$` branch is dropped, output unchanged
+* Allocate less in `RuleSet`: skip the downcased copy of already normalized property names, and build dimension shorthands without enumerators
 
 ### Version 3.3.0
 * Fix Arbitrary local-file read via attacker-controlled `@import`
diff --git a/lib/css_parser/rule_set.rb b/lib/css_parser/rule_set.rb
index 453c34f..cff4bd7 100644
--- a/lib/css_parser/rule_set.rb
+++ b/lib/css_parser/rule_set.rb
@@ -18,6 +18,8 @@ class RuleSet
 
     NUMBER_OF_DIMENSIONS = 4
 
+    NORMALIZED_PROPERTY_RX = /\A[-a-z0-9_]+\z/.freeze
+
     DIMENSIONS = [
       ['margin', %w[margin-top margin-right margin-bottom margin-left]],
       ['padding', %w[padding-top padding-right padding-bottom padding-left]],
@@ -205,8 +207,7 @@ def to_s(options = {})
           importance = options[:force_important] || value.important ? ' !important' : ''
           memo << "#{prop}: #{value.value}#{importance}; "
         end
-        # TODO: Clean-up regexp doesn't seem to work
-        str.gsub!(/^[\s^({)]+|[\n\r\f\t]*|\s+$/mx, '')
+        str.gsub!(/^[\s^({)]+|[\n\r\f\t]+/, '')
         str.strip!
         str
       end
@@ -226,7 +227,10 @@ def ==(other)
       attr_writer :declarations
 
       def normalize_property(property)
-        property = property.to_s.downcase
+        property = property.to_s
+        return property if property.match?(NORMALIZED_PROPERTY_RX)
+
+        property = property.downcase
         property.strip!
         property
       end
@@ -584,10 +588,11 @@ def create_dimensions_shorthand! # :nodoc:
       return if declarations.size < NUMBER_OF_DIMENSIONS
 
       DIMENSIONS.each do |property, dimensions|
-        values = DIMENSION_DIRECTIONS.each_with_index.with_object({}) do |(side, index), result|
+        values = {}
+        DIMENSION_DIRECTIONS.each_with_index do |side, index|
           next unless (declaration = declarations[dimensions[index]])
 
-          result[side] = declaration.value
+          values[side] = declaration.value
         end
 
         # All four dimensions must be present