Example benchmark:
$ time build/bench/bin/mruby -e "Array.new(2_000_000){ |i| i }.index{ |i| i == 1_999_999 }"

Before:
real    0m0.934s
user    0m0.922s
sys     0m0.003s

After:
real    0m0.590s
user    0m0.583s
sys     0m0.007s