Try compiling V8 for Mac with -O3 when using the v8.gyp file.

tools/gyp/v8.gyp

 # Copyright 2009 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 83 matching lines @@
                   '-fno-strict-aliasing',
                   # Avoid crashes with gcc 4.4 in the v8 test suite.
                   '-fno-tree-vrp',
                 ],
               }],
             ],
             'cflags_cc': [
               '-fno-rtti',
             ],
           }],
+          ['OS=="mac"', {
+            'xcode_settings': {
+              'GCC_OPTIMIZATION_LEVEL': '3',
+            },
+          }],
           ['OS=="win"', {
             'msvs_configuration_attributes': {
               'OutputDirectory': '$(SolutionDir)$(ConfigurationName)',
               'IntermediateDirectory': '$(OutDir)\\obj\\$(ProjectName)',
               'CharacterSet': '1',
             },
             'msvs_settings': {
               'VCCLCompilerTool': {
                 'RuntimeLibrary': '0',
                 'Optimizations': '2',
@@ skipping 461 matching lines @@
       ],
       'conditions': [
         [ 'OS=="win"', {
           # This could be gotten by not setting chromium_code, if that's OK.
           'defines': ['_CRT_SECURE_NO_WARNINGS'],
         }],
       ],
     },
   ],
 }