From the documentation of rb_obj_hash: > Certain core classes such as Integer use built-in hash calculations and > do not call the #hash method when used as a hash key. So if you override, say, Integer#hash it won't be used from rb_hash_aref and similar. This avoids method lookups in many common cases. This commit uses the same optimization in rb_hash, a method used internally and in the C API to get the hash value of an object. Usually this is used to build the hash of an object based on its elements. Previously it would always do a method lookup for 'hash'. This is primarily intended to speed up hashing of Arrays and Hashes, which call rb_hash for each element. compare-ruby: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux] built-ruby: ruby 3.1.0dev (2021-09-29T02:13:24Z fast_hash d670bf88b2) [x86_64-linux] # Iteration per second (i/s) | |compare-ruby|built-ruby| |:----------------|-----------:|---------:| |hash_aref_array | 1.008| 1.769| | | -| 1.76x|