zenprofiler helps answer WHAT is being called the most. spy_on helps answer WHERE those calls are being made. ZenProfiler provides a faster version of the standard library ruby profiler. It is otherwise pretty much the same as before. spy_on provides a clean way to redefine a bottleneck method so you can account for and aggregate all the calls to it. % ruby -Ilib bin/zenprofile misc/factorial.rb 50000 Total time = 3.056884 Total time = 2.390000 total self self total % time seconds seconds calls ms/call ms/call name 50.70 1.64 1.64 50000 0.03 0.05 Integer#downto 19.63 2.27 0.63 200000 0.00 0.00 Fixnum#* 14.19 2.73 0.46 50000 0.01 0.05 Factorial#factorial 9.93 3.05 0.32 1 320.36 3047.10 Range#each 5.54 3.23 0.18 2 89.40 178.79 ZenProfiler#start_hook Once you know that Integer#downto takes 50% of the entire run, you can use spy_on to find it. (See misc/factorial.rb for the actual code): % SPY=1 ruby -Ilib misc/factorial.rb 50000 Spying on Integer#downto Integer.downto 50000: total 50000: ./misc/factorial.rb:6:in `factorial' via ./misc/factorial.rb:6:in `factorial'