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"Why attr_reader and attr_writer are faster than def

VM_METHOD_TYPE_ATTRSET and VM_METHOD_TYPE_IVAR are the reason attr_accessor is significantly faster than a normal method created with def. These are implemented as pure C functions without creating a new Ruby stack frame, or executing compiled Instruction Sequences inside the VM."

I'll go edit the post to make it more clear that you need to follow the links through the code to get the actual answer... it's more involved than a single snippet.

I was about to make the same comment.

I'll take a wild stab at why setting/getting `b` is slower. The author of the code has just inserted another call layer by manually defining the getter and setter in Ruby.