> If JS engines were better at monomorphizing access to common subtypes across different map shapes maybe it'd be closer, but no engine has implemented that or seems to have much appetite for doing so.

I really wish JS VMs would invest in this. The DOM is full of large inheritance hierarchies, with lots of subtypes, so a lot of DOM code is megamorphic. You can do tricks like tearing off methods from Element to use as functions, instead of virtual methods as usual, but that quite a pain.