In mobile HTML5 apps _everything_ matters. I spent a significant amount of time building HTML5 apps that mirrored native UI almost perfectly and the primary issue with such an interface was that loading took forever the first time you open the page/app (but was natively-fast thereafter). If you're caching the app offline then JS performance (and moreso CSS performance) quickly become the bottleneck in app usability (because of the huge startup costs of setting up the page on the order of 10 seconds for something your desktop loads instantly).