This command repeatedly gets the specified process' stack using pstack (which is an insanely clever and tiny wrapper for gdb) and displays it fullscreen. Since it updates every second, you rapidly get an idea of where your program is stuck or spending time.
The 'tac' is used to make the output grow down, which makes it less jumpy. If the output is too big for your screen, you can always leave the 'tac' off to see the inner calls. (Or, better yet--get a bigger screen.)
Caveats: Won't work with stripped binaries and probably not well with threads, but you don't want to strip your binaries or use threads anyway.
watch -n 1 'pstack 12345 | tac'