benchmarks runing [$ whichr whichr] (the whichr gem just behaves like the "which" command in Linux), best time seen:

mingw 1.8 

  normal rubygems
    0.77

  faster_rubygems with cache
    0.14
    
  faster_rubygems without cache:
    0.20

mingw 1.9

  normal rubygems:
    3.79

  faster_rubygems with cache:
    0.19
    
  faster_rubygems without cache:
    0.50  

jruby, 1.8 mode

  normal rubygems:
    2.31

  faster_rubygems without cache (deleted the cache file temporarily to force it to not use it):
    1.15
    
  faster_rubygems with cache:
    1.11
    
    
rails 2.3.8 [ $ ruby script\runner -e production "puts 3" ] 
    Unfortunately rails requires a load of full rubygems.  So currently faster_rubygems doesn't help it much.

faster_require (other gem) seems to help, however.

1.9
    normal gem_prelude[RUBYOPT=]
      20.85

    faster_rubygems without cache [RUBYOPT=-rfaster_rubygems]
      20.7

    faster_rubygems with cache [RUBYOPT=-rfaster_rubygems --disable-gems]:
      12.10
      
    normal gem_prelude, with faster_require gem [RUBYOPT=-rfaster_require]:
      8.93
      
    faster_rubygems, without cache, with faster_require [RUBYOPT=-rfaster_rubygems -rfaster_require]:
      8.99 
      
           * recommend this configuration for ruby 1.9 and rails.
      
    faster_rubygems, with cache [RUBYOPT=--disable-gems -rfaster_rubygems -rfaster_require]
      fail (gotta fix that, then will probably be fastest option)
    
1.8

  normal rubygems:
    6.54
  
  faster_rubygems without cache:
    6.12
    
  faster_rubygems with cache:
    6.48
    
  normal rubygems, faster_require:
    3.53
    
  faster_rubygems with cache and with faster_require (have to use it as require 'faster_require')
    2.97
      
       * recommend this configuration, which means you'll have to add a require 'faster_require' to your rails script at the beginning, like in boot.rb
    
  faster_rubygems without cache and with faster_require
    3.31

    
TODO:
 jruby spawning itself was *so slow* it seemed (timer.rb)  
 only regenerate appropriate cache files after gem installs...I guess.
 only cache lib filenames, not spec files, et al...
 chmod after create (check if necessary first)