Microsoft have openly admitted that their own security software isn't as good as many third party offerings. Further the very worst culprit for hogging resources (on every computer I've ever had Microsoft Security Essentials or Windows Defender on) is a program called MsMpEng.exe, which is the 'updater' for Microsoft security software.
I gave up on Microsoft stuff years ago because of the very problem that you've been experiencing. Then I gave up on AVG for the same reason. That's why I use the free version of Avast. (I see no reason to use any paid-for program).