The problem is that developer has no idea if the implementation is going to be performant for their use case. The mutable compilers normally have less magic and

The problem is that developer has no idea if the implementation is going to be performant for their use case. The mutable compilers normally have less magic and thus are easier for the developer to understand and optimize. FP is really just a higher level language, that's why they don't teach algorithms classes using FP languages.