Yeah, your mileage will vary of course, and there’s no point comparing C++ to BP in terms of raw performance.
So, for the 9999 normal use-cases for blueprints, the answer to the original question is “don’t worry about it”. If you’re dealing with that single remaining performance-critical case, then C++ is the real answer.
I just ran some final tests with an automated frame counter and accumulating deltatimes to get a more accurate average, and the result was 90 ms for interface calls, and 86 ms for cast+call. That’s for 10 000 loops per frame, and I let it run for a total of 1000 frames per test to get the average to stabilize properly at a “true” value.
gives a difference per single call of a mere 0.4 microseconds (a.k.a. nothing).