The first time you call a method through an interface, .NET has to figure out which type and method to make the call on. It will first make a call to a stub that finds the right method to call for the appropriate object implementing that interface. Once this happens a few times, the CLR will recognize that the same concrete type is always being called and this indirect call via the stub is reduced to a stub of just a handful of assembly instructions that makes a direct call to the correct method. This group of instructions is called a monomorphic stub because it knows how to call a method for a single type. This is ideal for situations where a call site always calls interface methods on the same type every time.
… Coding For Performance : Interface Dispatch