C is far from simple, a simple language would be something like Scheme, Forth, or even SML. It so happens that the C standard (which is nearly 1k pages long nowadays) hides a lot of pitfalls, making the job of optimisers, implementators, and regular programmers miserable.
Are you sure about that? Forth is famous for being extremely fast. As for SML, assuming a variant with manual memory management (which I do not consider to add complexity in C), I can't think of any reason on why it would be more difficult to optimise.
The main reason a language like SML or Haskell is difficult to optimize is because it is incredibly difficult to canonicalize for the purpose of making peephole optimizations and abstract interpretation effective.
https://sunfishcode.github.io/blog/2018/10/22/Canonicalizati...
