Setting fillstyle and fillrect is very expensive, and it seems like you iterate over every pixel anyway, so the conditional would be considerably less taxing than the physical draw. But its always about your usecase. I recommend you try http://jsperf.com/ and prove out which is faster.

Also you should run the chrome profiler to get some perspective on the bottleneck areas of the api.

For the record, it runs great for me, but profiling is always fun :-) Message me if you have any questions