I was a Python coder myself. Normally PyPy is faster, but once I submitted a problem in PyPy and it got TLE (the same code was later accepted in Python3). So I had to decide which to use where.

Even though a solution is O(n) and optimal does not guarantees an AC in every language. Internally Python is slower than C++ and a more efficient solution in Python may get TLE whereas a less optimal solution may get an AC in C++. Even though there is a time relaxation in the background for some languages, that is not enough to get an AC.

Therefore sometimes it is written that_ a solution is not guaranteed in every language_.