I'm afraid that this is a bit like asking "Which is faster, a car door or a car door handle?" Just as it's the engine that moves the car, it's not the interface/class distinction that determines speed; instead, it's the underlying code.
Interfaces would, on average,
compile faster, since there's no underlying code to compile, but that's about it. Interfaces are an important part of desing, and
you should not shun them because of (unwarrented) fears over speed.
Piscis Babelis est parvus, flavus, et hiridicus, et est probabiliter insolitissima raritas in toto mundo.