The performance would be bad because of no JIT, but the main issue it is that Apple requires WebKit usage:
> 2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript.
> 4.7 HTML5 Games, Bots, etc. -> only use capabilities available in a standard WebKit view (e.g. it must open and run natively in Safari without modifications or additional software); and use WebKit and JavaScript Core to run third-party software and should not attempt to extend or expose native platform APIs to third-party software;
Now, I get their reasoning - if you did parse regular javascript and JIT was allowed (which would be required to prevent any anticompetitive remarks in terms of performance for these other engines), random pages could exploit those 3p browsers and extract information from the APIs they have access to.
But computers are complicated enough to where you probably couldn't explain to a layperson/congressperson the difference between iOS forcing developers to use their Window management APIs and iOS forcing developers to use their javascript APIs, so this is unlikely to change with regulatory force or anything of the sort.
I am technical and I still don't quite grasp the reasoning. Why does it matter if a browser makes available a non-standard API? And why is this a concern for mobile and not desktops?
Apple doesn't want app developers to deflect responsibility of user data siphoning off onto third-party websites or parties. Think the contacts API - the user might grant the app access to it in general, but then facebook.com could try to access them via a special browser API; if there aren't any extra prompts within the browser's code asking for user permission, this could lead to the site siphoning off user contacts without the user knowing.
> 2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript.
> 4.7 HTML5 Games, Bots, etc. -> only use capabilities available in a standard WebKit view (e.g. it must open and run natively in Safari without modifications or additional software); and use WebKit and JavaScript Core to run third-party software and should not attempt to extend or expose native platform APIs to third-party software;
Now, I get their reasoning - if you did parse regular javascript and JIT was allowed (which would be required to prevent any anticompetitive remarks in terms of performance for these other engines), random pages could exploit those 3p browsers and extract information from the APIs they have access to.
But computers are complicated enough to where you probably couldn't explain to a layperson/congressperson the difference between iOS forcing developers to use their Window management APIs and iOS forcing developers to use their javascript APIs, so this is unlikely to change with regulatory force or anything of the sort.