I gave this a brief test. I typically got numbers around 0.5ms, although it fluctuates quite a bit between 0.0 and 1.0. It rarely goes above that, and the maximum I've seen it go to is 1.9ms. Tested in Firefox 20.0.1. There was just my character on the map at the time.
I presume there'd be more lag once the maps get larger and use more graphics/tiles (or will they be pre-rendered?).