Loving it so far, needs to be able to run in the background tho, currently only works when its the active page being viewed (at least on the web version) not sure about the non web version.
That's a feature of your browser, actually. It'll slow down javascript on non-active tabs. You can adjust it in your advanced config settings (go to "about:config" as a url if you're on firefox), but it's *generally* not recommended and is thus a rather arcane thing to adjust. You can avoid most of it by opening a new window with the game as the only tab, or just run the desktop version.
Yeah this is a bit of a necro, but figured it'd help to put the answer here!
As a Linux user that extensively uses different workspaces, if you have the game up in a browser on a workspace of it's own then it does not get slowed down if you switch to different workspaces. I know there are numerous aftermarket versions of workspaces that can be added to Windows, not sure if some of these will work the same way or help with this issue there.