gistfile1.txt

$ cat Gemfile
source 'https://rubygems.org/'

$ time ruby -e 'puts "Hello world"'
Hello world

real	0m0.197s
user	0m0.117s
sys	0m0.073s

$ time bundle exec ruby -e 'puts "Hello world"'
Hello world

real	0m1.046s
user	0m0.863s
sys	0m0.172s

If you ever wanted a reason to use bundler's binstubs:

this is in a reasonable size rails 3.2 app, the $PATH includes the binstubs directory

$ time rake environment
noglob rake environment  0.09s user 0.02s system 16% cpu 0.637 total

$ time bundle exec rake environment
bundle exec rake environment  5.56s user 1.20s system 99% cpu 6.795 total