Pretty much every method call, unless it is optimized—will go through `objc_msgSend`. The runtime looks up the code for a particular object using `objc_msgSend.` The function signature varies depending on how many parameters the method has but it always has at least 2 arguments—the instantiated object and the method name. Objc calls this the selector. :)

If you write a message send in Objective-C, like [object doSomething], behind the scenes the compiler translates this to a call to the objc_msgSend function in the Objective-C runtime library. This function looks up what code needs to run, taking into account inheritance and all the other dynamic features and runs it. So this function runs really often and is an important target for optimization.

It is a codepath that is used a lot, in system frameworks that many apps use. So, speeding it up can yield a systemwide performance improvement for all sorts of apps.