The UI should feel spry and peppy. Animations should zip. Screens should respond immediately to the user's touch. If there's a reason people love their phones so much, it's because mobile apps are very good at delivering on these promises.
Historically, though, hybrid and web apps have lagged behind their native counterparts in the "peppy" department. Mark Zuckerberg famously griped that HTML5 "just wasn't ready" – a phrase that still echoes in the minds of many mobile developers today.
For the mobile web to be taken seriously, it needs to provide the same smooth performance that we've come to expect from native apps. In short, it needs to close the UX gap.
I recently gave a talk at Brooklyn JS where I shared some tricks for building web and hybrid apps with near-native performance. It wasn't recorded, but you can read the full slides and speaker notes online.
The slides themselves are kinda neat, because they contain the very CSS animations that I talk about in the presentation. So you can open it up on a mobile browser and see the difference between, say, hardware-accelerated and non-accelerated animations. The FPS demo is also fun to play with.
Just don't judge me too harshly if you open the slides in IE or Safari – I didn't have enough patience to get WEBM video or flexbox working properly. The CSS animations should look lovely, though.