it is because chrome slows main thread js execution for inactive tabs. The solution is to do your scheduling (setInterval etc) in a web worker.

Yeah, that's a common Web Audio hassle. On mobile it gets even crazier, although I haven't tried this site in mobile.

For me, in Firefox on Yosemite, the slowdowns seem to come at random, even when the tab is in focus.

A workaround is to detach the tab and keep it visible. Then it doesn't need to be focused.

Quite nice live gadget!