Check to see what is running in the background. It may be that your computer is downloading updates to your OS, to your anti-virus, to your browser(s) & their plugins, or to your apps. Or it may be updating the search indexes in the background. All of these things can take up your CPU & all but the search update also take up your ISP bandwidth which you may notice as lag. Much of this stuff has a delayed start on boot-up which could be why your game works fine if you start it after a fresh boot. So ctrl-alt-delete & start task manager & see what is using your CPU when things are lagging, then, if needed, do an internet search to figure out what process it is and how you can stop or reschedule it.
I disable automatic updates & even delete the automated updating Google Chrome & some others slip in to the Services. Of course that means I have to take the responsibility to update things manually on a periodic basis.