Whether it is a bottleneck or not is a different question.

I think mpweiher's point was that in normal Objective-C applications, the fact that message sending is slow is irrelevant. Yes, it is slower than a C function call, but it's rare that it is an issue in real code. When it was a problem before, in something unusual like your article's heavy tree traversal, you'd inline the code or drop down to C. But that was fairly rare. Apple has also improved objc_msgsend within the past few years.

My wild guess is that with Swift, Apple is trying to get a more lightweight language not due to performance issues but due to battery consumption. I think that around 40% of CPU time of your average obj-c app is spent in objc_msgSend. That is potentially a lot of power essentially wasted.