It depends. There is always an increased demand for memory (which can impact the whole hierarchy) but other than that it shouldn't be bad. And I think the point is that compared to interface{}, generics will almost always win. And realistically, compared to hand-rolled, you'd need to hand-roll (or codegen, whatever) multiple implementations which, as machine code, would end up exactly the same as the generic implementation.

And what are we optimizing for anyway? Reading a bunch of code that uses an off-the-shelf library's Stack or Tree or whatever is always going to be easier to read than having to also have to inspect the three hand-rolled Tree implementations, two of which are probably buggy, in addition to the actual client code trying to use the tree to do real work. Or a library that implemented it using interface{} which is a big middle finger to the whole type system, completely discarding the compiler's ability to detect otherwise trivial mistakes.