overhead of starting threads

[Simon Percivall]

How much you gain by starting threads is also determined by what you're
doing in those threads. Remember (or learn): In CPython only one thread
at a time can execute python code, so depending on your task threading
might gain you little. If you're doing I/O or calling functions written
in C (and if they release the Global Intepreter Lock [GIL]) you might
gain a lot by using threads.

As for overhead: profile, profile, profile. You'll have to do sample
runs and find your sweet-spot. It all depends on what you're doing in
the threads.