The Lua C implementation seems very conservatively written for portability and maintainability, but it's not slow either.
Handwritten assembly really can be faster than compiler generated code. The proof is that we can always look at the output of the compiler and invest more time improving on it by hand, whereas the compiler is required to complete in a short amount of time and usually without actually timing its code on the target machine.
Now if you take someone experienced in hand-tuning assembly like that and ask them to write the fastest possible code using a compiler, they're going to beat the pants off an ordinary coder who hasn't been benchmark everything he writes all along.
But the real lesson here is that Lua is just freaking awesome.
Handwritten assembly really can be faster than compiler generated code. The proof is that we can always look at the output of the compiler and invest more time improving on it by hand, whereas the compiler is required to complete in a short amount of time and usually without actually timing its code on the target machine.
Now if you take someone experienced in hand-tuning assembly like that and ask them to write the fastest possible code using a compiler, they're going to beat the pants off an ordinary coder who hasn't been benchmark everything he writes all along.
But the real lesson here is that Lua is just freaking awesome.