I've had much grief with this in Rust, adding to an array with "push".

Down at the bottom, this grows by resizing an allocation. Ordinarily, that's cheap. But if you're resizing a reasonably sized array, at least under Wine it's a copy of the array with the lock on the memory allocator set. There's a spinlock involved. If many threads are allocating, futex congestion is possible and speed drops by two orders of magnitude. Ouch.