The bam loader will be faster because the bam loader is not written in Python.
It’s not the addDataX calls themselves that are slow; it’s the overhead of calling them from Python. Python is an excellent language for doing the high-level manipulation of your scene, like moving objects around; but it’s not so great at doing things on a per-vertex level, simply because the overhead of a single Python function call tends to dominate when you end up calling the same function repeatedly many thousands of times.
Your two choices for improving performance are (a) moving the thousands of function calls into C++, or (b) not making thousands of function calls.
David