The security update KB2267602 (Version 1.363.1278.0) for Microsoft Defender dropped yesterday and I installed it today. Ever since I did, I saw random slowing down of the machine, something to do with graphics, I think, because games suddenly run at 15 fps when they used to run at well over 65 and even scrolling is becoming very laggy. At times, all the animations for the OS are very jagged, as though running on a GT 210 (Though I know they couldn't). I don't know if this has to do with the new update, it's interaction with the GPU driver or if the new update recognises my game as malware. Whatever it is, I do hope Microsoft will patch this issue very soon. Thank You.
Are you using insider build or stable build? Try update Microsoft Defender and run a full system scan with it. Restart your PC. If problem persist, open start and search for feedback and open the Feedback Hub app and report this issue.