Do you mean because it already did the check before, and it could remember the results? Certainly it could. Unfortunately this isn't an option in Objective-C because the compiler can't make any assumptions about the tagged pointer implementation, and that means objc_msgSend has to start fresh at each invocation. In theory you could have sub-functions, like objc_msgSend_really_object and objc_msgSend_tagged_pointer that the compiler could generate calls to directly, but then you'd have to support whatever assumptions the compiler made forever.

I'm guessing/hoping that with modern branch prediction, the check at the top of objc_msgSend is a tiny penalty. I'm sure Apple measured the heck out of it, anyway.