I know that it freezes the classes (which are also objects, indeed). And I saw it does this by reading classnames from a list of textfiles. Both is not fast. The "freeze" isn't that invasive, from what I can see in the c code, it merely flips a flag at the objects metadata table. But I can imagine the reading from files isn't that fast.

And tracking the mutation flags isn't free either. Though I expect not that big. Checking for the state of this flag at every possible mutation is quite some overhead, I'd presume. But no idea how heavy that'd be.

Again, compared to typical boot stuff like Rails' zeitwerk autoload mapping, it's nothing. And in runtime the only penalty will be when something tries to mutate a class obj, which I'd expect happens almost only on boot in most codebases anyway.

Though I know quite some "gems" and even older Rails implementations (mostly around AR) that would monkeypatch in runtime on instantation even. Like when a state-machine gets built on :new or when methods get added based on database-columns and types, every time such an instance is returned from a db response.