I've always avoided the physics plugin on mobile games because a few years ago it was just unplayable.
Now that some years have passed, browsers have matured and I have two "modern" phones, I decided to try again.
I used 2 phones released in September 2013: Xiaomi Mi3 (Snapdragon 800 2.3Ghz with android 4.4) and iPhone 5s.
On both phones (Chrome and Safari) I was very surprised to see that with 100 physics objects and 4 immobable objects the framerate stays at 60fps, even without WebGL it's still almost 60 with some drops. I know these phones are still pretty good today, but 100 objects is a little overkill for a game so I think even phones with a lower cpu will perform well with less objects.
Also good results on Xiaomi default browser, on Firefox it drops a lot more but I've always had performance problems with firefox and mobile.
Overall I'm pretty happy with how good html5 has become already. What do you think?
I'll leave the test I used in case you want to try yourself, just touch around to apply a force:
https://dl.dropboxusercontent.com/u/1910765/physics_test/index.html