> It was always a dumb choice to make member functions default to virtual semantics, when they almost always don't need it, and it just costs performance to no purpose.

If you actually don't need it (i. e., your hot methods are never overridden), then the JIT will trivially compile those "virtual" method calls as non-virtual ones. It has all the information it needs, since it knows what classes are loaded. It can invalidate its code if necessary, if you load more classes that do add overrides. So no, it does not cost performance at the call site. It does cause the compiler to do work, but nothing fancy.