[ant6n]

I found that PyPy sometimes has unexpected slowdowns. When we were porting from Python to PyPy on some offline processing tools, the most crazy one was building strings via += and sum(arrays,[]), which is much slower than cpython.

[Fede_V]

There was a good blog post this by Armin. Basically, if you have to concatenate strings, don't do so via += but use "".join([]).

[ant6n]

I find that unexpected. Java has had string builder optimization for a long time, and CPython is much much faster in this respect. It's not always easy to use "".join when using a string, so you end up having to build a separate array of strings in some cases. And building arrays isn't always that fast either. And [].join doesn't exist, so summing arrays is always kinda slow.

Anyway, all that is to say: I really like PyPy, and we use it a lot, but those _unexpected_ crazy slowdowns are unfortunate.