>Its one reason I struggle with the modern fashion for immutable classes and FP, they are always making copies of everything, seems crazy

It depends how it's implemented. It's possible to get very nice performance with immutability and copying through use of an arena allocator, as your stuff will essentially always be in cache (due to reusing the arena), and allocation/deallocation is just bumping a pointer. Of course, not everything easily fits into this approach, but a surprisingly large amount of code can, if designed with it in mind (and using a language that supports it without too much pain, like C/C++).

The language Zig is particularly interesting in this regard because everything that allocates takes the allocator as a param, and it has built-in arena allocators in the standard lib.