Hard to tell what it might be, but there's a couple of things easy enough to try.
The first suspect for me is the bunch of Windows Apps. They're allowed to hog a serious amount of resources for updates but that doesn't necessarily show as a constant high percentage in the Task Manager. Uninstalling those that a) you don't use and b) that can be uninstalled never harms anyone. There's new ones after the latest updates so going through Apps and Features might be an eye opener.
While you're at it, go to the Settings > Privacy > Background apps and disable the entire bunch. I haven't noticed any disadvantage from having it off since I started using Win10. Further, even Microsoft hints it's optional as they suggest turning apps off for longer battery life.
In Settings>System>Notifications and actions uncheck every tick box and turn off every feature you don't feel like wanting announcements from. AutoPlay is the only one I like to keep as it tells when it's safe to pull a USB stick out.
The new Bing that wants to sit either in the middle of your Desktop or at least populate the Search bar can also be the culprit. I don't know much about it yet but it seems to be very persistent. Disabling the Search Highlights is also reccommendable - what exactly is the idea of bringing up random landscapes or animals? Are they trying to educate us?
Last but not least, check your Edge settings. First and foremost disable the Startup Boost and also turn off the 'keep running in the background when closed'.
That's NOT all, Folks!
