Older devices will slowly dissipate from the market because of the ubiquitous two-year contracts, though, and eventually everyone's phones will be capable of delivering native-like performance on HTML5 apps. It's only a matter of time.
The moment older devices slowly fade out from the market, and new high end devices started to fade in, native app speed now doubled.
Doubled from awesome to... awesome? Native performance is the goal. I don't see how much better a data heavy native app like FB can get, but I do see how HTML5 apps can get better.