How to find out why a program is running so slow?

I have a program running really slow. I found that actually a function takes long time. But by debugging I don't know how to identify which piece of code within this function causes such slowness. Any guru here can tell me the best way to find out why? Thanks for your inputs.

Run a profiler on the app.

Profiling will give you a detailed analysis, but you could always just step through your program and observe which function calls take a long time to return.