OK, just did a little test. Opened Device Manager with no other apps running (aside from Windows and background drivers) with WU off. CPU usage was 0-2%. So I set WU to check but not install and CPU usage went up to 50-75% instantly and stayed up there. After I opened IE11, CPU usage pegged at 100%. Turned WU off and CPU usage went back to 0-2% and only to 7-15% with IE11 running. So I have to conclude -at least here- WU is causing some of the problem. I'll leave it set to never and I don't think it hurts to leave MsMpEng,exe in the excluded zone, unless somebody can tell me if there's a reason to think otherwise. THANKS!  
MsMpEng.exe (Microsoft Security Essentials) has suddenly / recently been using a lot of CPU time, to the point that performance is lagging (Win 7 Pro+SP1), especially for IE11. If I open MSE control panel and turn off real time protection, CPU usage drops. I can turn real time protection back on as soon as CPU usage drops, and things are ok for a while... Don't have any MSE scans scheduled so only real time scanning is active. I've seen posts suggesting to add C:\Program Files\Microsoft Security Client\MsMpEng.exe to the list of excluded files and locations as the fix for this. I would think this should only have any effect during an actual scan, but not during "real time protection". SAS and MSE don't find any malware so I don't believe that's the problem. And MSE is fully up to date. Thoughts and Ideas?
MsMpEng