I'm mostly just hoping it won't become necessary, though that is perhaps a vain hope.

The reasoning is that, according to my interpretation of talking with some folks working on JSC and SM, property lookup inline caching is the most important performance optimisation bar none. JIT compiling is an improvement on top, definitely, but it is not an massive step change.

Safari browser has a no-JIT mode that is fairly widely in use, and it is apparently fast enough that you don't really notice the change. Ladybird browser's LibJS has no JIT compiler, yet LibJS isn't really unbearably slow: The browser's biggest performance woes come from the browser around it and especially from having the simplest possible drawing algorithm possible.

From a "personal" experience, while the test262 compliance test set is no performance benchmark, Nova is for some reason consistently at the very top of the runtime list over at https://test262.fyi/#. This is of course partially just because we're really quick to do a controlled panic if an unsupported code path is called, and the remaining part is because the code is run so little that JIT doesn't get to kick in. Still, this meaningless number gives me some measure of hope: We're consistently 3 times as fast as V8 after all :)