bench.rb

require 'benchmark'

class Hash
  def orig_symbolize_keys!
    keys.each do |key|
      self[(key.to_sym rescue key) || key] = delete(key)
    end
    self
  end

  def symbolize_keys!
    keys.each do |key|
      self[(key.to_sym rescue key)] = delete(key) if key.respond_to?(:to_sym) && !key.is_a?(Fixnum)
    end
    self
  end
end

HASH1 = { 'a' => :a, nil => nil, :b => :b, 1 => 1, [] => [] }

HASH2 = { 'a' => :a, :b => :b, 1 => 1, [] => [] }

TESTS = 100_000
Benchmark.bmbm do |results|
  results.report("Hash#orig_symbolize_keys! , nil key") { TESTS.times { HASH1.orig_symbolize_keys! } }
  results.report("Hash#symbolize_keys! , nil key") { TESTS.times { HASH1.symbolize_keys! } } 
  results.report("Hash#orig_symbolize_keys!") { TESTS.times { HASH2.orig_symbolize_keys! } }
  results.report("Hash#symbolize_keys!") { TESTS.times { HASH2.symbolize_keys! } } 
end  

faster-hash-symbolize-keys.diff

From eaad3a8ad25fbea9c6e8c252ac304f57a9e8c53e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lourens=20Naud=C3=A9?= <lourens@methodmissing.com>
Date: Mon, 8 Feb 2010 00:05:33 +0000
Subject: [PATCH] Hash#symbolize_keys(!) optimizations

---
 .../lib/active_support/core_ext/hash/keys.rb       |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/activesupport/lib/active_support/core_ext/hash/keys.rb b/activesupport/lib/active_support/core_ext/hash/keys.rb
index 045a694..e4d429f 100644
--- a/activesupport/lib/active_support/core_ext/hash/keys.rb
+++ b/activesupport/lib/active_support/core_ext/hash/keys.rb
@@ -22,7 +22,7 @@ class Hash
   # to +to_sym+.
   def symbolize_keys!
     keys.each do |key|
-      self[(key.to_sym rescue key) || key] = delete(key)
+      self[(key.to_sym rescue key)] = delete(key) if key.respond_to?(:to_sym) && !key.is_a?(Fixnum)
     end
     self
   end
-- 
1.6.4.4

results.txt

methodmissing:~ lourens$ ruby hash_bench.rb
Rehearsal -----------------------------------------------------------------------
Hash#orig_symbolize_keys! , nil key   3.500000   0.160000   3.660000 (  3.686203)
Hash#symbolize_keys! , nil key        0.510000   0.000000   0.510000 (  0.506390)
Hash#orig_symbolize_keys!             2.000000   0.090000   2.090000 (  2.096217)
Hash#symbolize_keys!                  0.470000   0.000000   0.470000 (  0.468481)
-------------------------------------------------------------- total: 6.730000sec

                                          user     system      total        real
Hash#orig_symbolize_keys! , nil key   3.520000   0.160000   3.680000 (  3.753897)
Hash#symbolize_keys! , nil key        0.510000   0.000000   0.510000 (  0.513103)
Hash#orig_symbolize_keys!             2.020000   0.090000   2.110000 (  2.115938)
Hash#symbolize_keys!                  0.470000   0.000000   0.470000 (  0.481901)