[devld]

> I just assumed that readonly properties will always be pretty performant in general.

This is also the case for variables. var is faster than const, because for const (and let) the engine must do additional work (related to enforcing block-level scope, if I am not mistaken).

[WorldMaker]

To my understanding today it depends on the JS engine and more importantly what JIT stage that code is in. JITs optimize const/let in ways that they can't optimize var now.

Which is to say that if someone tells you that you should in general never use const/let for "performance" they are probably wrong, but yes there probably are still edge cases and microbenchmarks that make var look faster on some engines.