You could craft hand written assembly code which will be faster than optimised C code most of the time yet you don't. Plenty of programmers are perfectly writing imperative code in Java doing a ton of necessary boxing and unboxing. It's all a trade off between performance and usability. The fact is that the Ocaml compiler does a good enough job with functional code that its performance is actually comparable to imperative solutions most of the time.

This is called Single Static Assignment form and its denial of mutation is crucial to optimization, from common expression removal, to efficient register allocation, and all sorts of control flow analysis.