diff --git a/lib/unicode/display_width.rb b/lib/unicode/display_width.rb
index a95a77e..48381b8 100644
--- a/lib/unicode/display_width.rb
+++ b/lib/unicode/display_width.rb
@@ -61,8 +61,9 @@ def self.of(string, ambiguous = nil, overwrite = nil, old_options = {}, **option
 
       width = 0
 
-      unless options[:overwrite].empty?
-        width, string = width_custom(string, options[:overwrite])
+      overwrite = options[:overwrite]
+      unless overwrite.respond_to?(:empty?) && overwrite.empty?
+        width, string = width_custom(string, overwrite)
       end
 
       if string.ascii_only?
@@ -118,8 +119,8 @@ def self.width_custom(string, overwrite)
       width = 0
 
       string = string.each_codepoint.select{ |codepoint|
-        if overwrite[codepoint]
-          width += overwrite[codepoint]
+        if overwrite_width = overwrite[codepoint]
+          width += overwrite_width
           nil
         else
           codepoint
@@ -211,7 +212,7 @@ def self.normalize_options(string, ambiguous = nil, overwrite = nil, old_options
         raise ArgumentError, "Unicode::DisplayWidth: Ambiguous width must be 1 or 2"
       end
 
-      if overwrite && !overwrite.empty?
+      if overwrite && (!overwrite.respond_to?(:empty?) || !overwrite.empty?)
         warn "Unicode::DisplayWidth: Please migrate to keyword arguments - overwrite: #{overwrite.inspect}"
         options[:overwrite] = overwrite
       end
diff --git a/spec/display_width_spec.rb b/spec/display_width_spec.rb
index 067dd97..7afdfcc 100644
--- a/spec/display_width_spec.rb
+++ b/spec/display_width_spec.rb
@@ -3,6 +3,24 @@
 require_relative '../lib/unicode/display_width/string_ext'
 
 describe 'Unicode::DisplayWidth.of' do
+  describe 'overwrite fast path' do
+    it 'skips custom processing for omitted, nil, false and empty overwrites' do
+      expect(Unicode::DisplayWidth).not_to receive(:width_custom)
+      expect(Unicode::DisplayWidth.of('ABC')).to eq 3
+      [nil, false, {}, double(empty?: true)].each do |overwrite|
+        expect(Unicode::DisplayWidth.of('ABC', overwrite: overwrite)).to eq 3
+      end
+    end
+
+    it 'applies non-empty hashes, procs and lookup objects' do
+      lookup = double(empty?: false)
+      allow(lookup).to receive(:[]).with(65).and_return(4)
+      [{65 => 4}, ->(codepoint) { 4 if codepoint == 65 }, lookup].each do |overwrite|
+        expect(Unicode::DisplayWidth.of('A', overwrite: overwrite)).to eq 4
+      end
+    end
+  end
+
   describe '[east asian width]' do
     it 'returns 2 for F' do
       expect( '！'.display_width ).to eq 2