High level languages are easier to optimize because they are starting with more de-optimizations. Haskell can be optimized to perform stream operations on unboxed integers, precisely because integers are boxed to start with! Whereas the code you would have written in C to do the same thing would already be using unboxed integers, no?
The greatest value brought by compiler optimisations is removing the overhead of convenience. Sometimes that is about avoiding the boxing that is a necessity in many high level languages, but in other cases it serves to allow a more modular programming style without overhead. Stream fusion is a good example: it lets you structure your program as small and composable units, without the cost of manifesting intermediate results. That is not merely about avoiding the inherent inefficiency of e.g. Haskell, but about permitting different styles of programming, and the argument is that a low level language simply cannot allow such a style (without overhead), because the required optimisations are not practical to implement.
you can imagine a version of Haskell that doesn’t have polymorphism or laziness, that only has unboxed integers, and as a result doesn’t need a GC. in such a language it’s still easy for the compiler to do stream fusion; whereas it’s still hard in C because the compiler needs to prove the loop doesn’t have side effects.
