How does Objective-C selector lookup evolve from slow path to fast path?

Selector lookup starts with cache miss, falls back to method lists, and eventually resolves into cached IMP for fast execution.

When objc_msgSend is invoked, runtime first checks the class cache (fast O(1) hash lookup). If missing, it searches method lists in the class and superclasses (slow O(n)). Once found, the selector-IMP mapping is stored in cache. Over time, frequently used methods become extremely fast due to cache locality. This is why hot paths in UIKit feel fast despite dynamic dispatch.