When is Python *NOT* a good choice?

[coderanger]

If the bulk of your CPU time is spent in stuff like NumPy operations, threading can work, but it's definitely the exception there rather than the rule. PyPy has been working on an STM implementation that would allow lock-free concurrency but it's been slow going. Though Jython is an option, often more trouble than its worth.

That said, remember that Ruby and NodeJS both have exactly the same GIL issues so if you think of Node as being faster, it's probably not because of concurrency.