first, do you see anything obvious in the console ?
second, can you try commenting out the draw or the update and see if one or the other is slow, I usually comment out blocks of code to see what's going on.
third, you can use a profiler from xcode to see what is taking the most time.
common culprits for slow code:
a) loading assets in update or draw instead of setup
b) memory leak
c) some OS specific code that might be blocking
d) uninitialized variable that gets a bad value based on memory
you can also post your code or the smallest possible slow example and folks here can take a look.