why abstract class is faster than interface

When you call an abstract method or an interface method, the actual method to be called is not determined until runtime. The CLR then looks at the actual type of the object which is making that call (not necessarily the same type as the variable it's assigned to) and calls the version of the method which is defined in that object's class. This 'late binding' is done through a structure known as a 'vtable' (or virtual method table) which is also used when calling virtual methods 

Because a class can only inherit from one abstract class but can implement multiple interfaces, I have seen it suggested that the CLR may take longer to bind an interface method than an abstract method.

However, in modern conditions, such differences will be negligible and should certainly be ignored when designing your class hierarchy.