This problem is the responsibility of the OS scheduler. One of the most important goals of any scheduler should be to give foreground UI processes that spend a

This problem is the responsibility of the OS scheduler. One of the most important goals of any scheduler should be to give foreground UI processes that spend a lot of time sleeping in event loops priority. So if your OS scheduler is properly designed (as the ones in all major OS's are) a threaded browser shouldn't interfere with the system's responsiveness.