> - Message passing in ObjC is so optimized that it probably can't get much faster, and anyone acting like it's slow has a potentially skewed understanding of this

The problem with Objc message passing is not the absolute execution time involved in passing a message. The problem is that the dynamism involved completely disables the compiler from performing any code-inlining. Typically inlining small functions is what enables the compiler to unlock further optimisations. From simple things such as merging duplicate loads from same address, to auto-vectorization.