Which is faster Abstract class or interface and why

If you're talking here about execution speed, then an interface has none at all because it can't be instantiated and can't contain executable code of any description.

Although an abstract class can't be instantiated it may contain executable code including static methods/properties and an instance constructor which is called by its base classes. It's execution speed will therefore depend on the speed of such executable code and whether and to what extent it's invoked within the application.