Long post. What you're saying may have some good to know particulars that I didn't get right. But the net is: the only performant way to run JS or any other non-pre-compiled (jit) language is banned. And there's separate bans altogether on browser engines even if running like dogs it were tempting. Because Apple sucks & is mean & a shit.
These are both horseshit fuckery excuses. Cast these wads into the pit. It's all fuckery, & there's so many other security defense layers. These prohibitions are bad spirited jerks being bad and jerks.
Hah sorry for the wall of text. Totally agree the real reason Apple bans browsers is not motivated by security, and them pointing at the JIT/performance is just the one plausible excuse.
But besides alternative browsers, IMHO this ban does have valid security/privacy/anti-scam reasoning. If apps could mark pages of memory as executable, they could introduce arbitrary new behavior whenever they want. Though it sounds like iOS security is good enough that even than an app can't break out of its sandbox.
But even with the sandbox, there's no point in the App Store review process if apps can completely change their code/behavior whenever. This dovetails into an App Store debate, but I think there is value in the idea of a review process. I'm sure there'd be way more predatory and malicious scam apps out there taking advantage of the tech-illiterate without app reviews. (Though I'm not saying Apple always does reviews right, or that they shouldn't allow app side-loading with less stringent requirements). Whoops wrote another wall of text.
These are both horseshit fuckery excuses. Cast these wads into the pit. It's all fuckery, & there's so many other security defense layers. These prohibitions are bad spirited jerks being bad and jerks.