If you know that a method isn't virtual, calling that method is just a function call with an extra argument for 'this'. If it's virtual, you have to look it up in a vtable, probably dereferencing two pointers in the process and making it harder on your instruction fetcher.

Java makes a lot of common classes final (e.g. String and Optional) so that it can avoid this. As I understand it, it's also got a lot of pixie dust to predict this some of the time, but then you can't rely on it.