A friend who’s a software developer boils down that issue as follows:
- Nitro is a JIT engine and requires that an executable using it be able to mark data memory pages as executable. Safari is a system app so it gets to do that (there’s a signed entitlement in the app that allows this).
- Third-party apps using UIWebView (e.g. PhoneGap apps) do not get that capability so they do not get Nitro and I wouldn’t expect them to, since permitting marking data as executable makes app store review kind of pointless.
- Web apps running from home screen but not in full-screen mode (which launch inside Safari) run Nitro fine.
- Web apps running from home screen in full-screen mode launch inside Web.app, which is a system app and in theory should be able to run Nitro but it’s not because it lacks the entitlement.
Sounds like a bug to me.
For the discussion behind those points, see this thread at Hacker News.