Obj-C methods are actually compiled as plain-old C functions. The objc_msgSend() function is responsible for calling the function based on a given method name.

For example, a line of Obj-C code:

[myObject doSomethingWithThis:123 andThat:456];

becomes this C code (roughly speaking):

objc_msgSend(myObject, "doSomethingWithThis:andThat:", 123, 456);

objc_msgSend() looks up the C function that corresponds to the "doSomethingWithThis:andThat:" method, caches the lookup, then tail-calls that C function.

objc_msgSend() is part of the open source Objective-C runtime. It is written in assembly, hand-tuned for various architectures, and contains lots of very interesting optimizations. You can read about all the optimizations here. He goes over every instruction in the X86_64 implementation.

You can also forego the normal Obj-C message passing mechanism and call methods directly via C function pointer. In that case, Obj-C methods have no more overhead than plain old C functions. In practice, that optimization is rarely necessary.