Antimalware Service Executable hogging cpu every time I log in

that's defender scanning, and right after login is when it likes to do it. if it settles down after 10-15 minutes just leave it. if it's every login for ages, it's usually chewing through some huge folder - a game library, or a dev folder with thousands of tiny files. add that folder as an exclusion under virus & threat protection settings, exclusions. don't switch defender off unless something else is replacing it.