And C++ don't helps to speed up engine
(just kidding as Lua user mainly)
But im' not so wrong, if some stuff is slow or not that fast, you can have C++ it won't make it.
If another engine uses Lua, but all GUI, Physics are fully optimized and fast, indeed at the end, the game will be faster in that one than the other C++ with problematic physic component integration.
As i will stay with simple characters in LE3 for the moment, it's ok, but if i use more physics indeed i'll be in your not so good same position with engine and physics