So iiuc:

The libc++ implementation is bad. The libstdc++ implementation is fine. The issue with the former is that it doesn't have enough spare capacity so it has to shift elements around too often.

Actually I think the push_front is even worse than stated: O(n). Consider an array with capacity N+1, contains N elements. Now if you alternate push_front and pop_back then every push_front will cause memmove of N elements.

Oh and to make like a 4th addition to this comment: It's kind of funny that the code is so unreadable that the author found it more helpful to look at the generated assembler code.