Maybe it's Linux specific? I managed to run pyperformance and got this: ### python_startup ### Mean +- std dev: 23.2 ms +- 0.8 ms -> 23.4 ms +- 1.2 ms: 1.01x slower Not significant Note, I am not dismissing the report -- in fact it looks quite bad. But I am failing to reproduce it, which makes it harder to understand the root cause. :-( Maybe we can create a microbenchmark for this that just parses a large amount of code? Anyway, here's a random thought about why this might have such a big impact. Look at this snippet (found all over the parser.c file): if (p->level++ == MAXSTACK) { p->error_indicator = 1; PyErr_NoMemory(); } if (p->error_indicator) { p->level--; return NULL; } This is two "unlikely" branches in a row, and the first sets the variable tested by the second. Maybe this causes the processor to stall? Also, maybe it would be wiser to use ++X instead of X++? (Though a good compiler would just change X++ == Y into ++X == Y+1.) Anyway, without a way to reproduce, there's not much that can be done.