html5 isn't optimised as well yet, it will be a lot better when they sort out the little niggles.
Html5 runs slower than Java for me. I can get decent fps on html5 (20-30) but Java gives me 50 constant. Hopefully as they continue to update the html5 version my fps will improve and I can use it instead of Java.
RS3 is playing via a client for me, so I'm guessing it's java, do you have to run the webclient for the HTML5 version? I've had no problems with lag so far, regardless.

