How does the Objective-C runtime handle method caching and why is it critical for performance?

The runtime caches method lookups to avoid repeated expensive class hierarchy traversal.

When a method is first called, objc_msgSend performs a lookup in the class’s method cache. If not found, it walks the class hierarchy. Once resolved, the selector-to-IMP mapping is stored in a hash table cache. Future calls hit this cache, reducing dispatch from O(n) inheritance traversal to O(1). Cache misses degrade performance significantly in dynamic-heavy apps.