I think that complaint applies to the (vast, essential) ecosystem, not to the language and standard library. AFAIK, there aren't serious performance potholes to hit in the language and standard library any more. You're not paying a huge performance penalty for writing things the simple way, but things are uniformly pretty slow.

It's when you pull in fast libraries that composition breaks down e.g. the many-fold difference between numpy's sum method and summing a numpy array with a python for-loop and an accumulator.