I have been running without AV protection for over a year - no problems but I'm pretty hep on how to avoid the gremlins. My bootup time is always less than 45 seconds from POST to full Desktop and network support components fully loaded.
Now, when I read up on MSE, it sounded so good I thought I'd give it a whirl. Not so much for my own use but for those people that I guru for. It sounded like an excellent program since it removed the user responsibility for updating and scanning. My people aren't easily trained so I was eager to find something that covered for their neglect.
I tested MSE for a full week. First downside, it jumped my bootup time to 1 min. 30 seconds. I can live with that, especially if it's on someone else's computer.

Next downside, it evidently doesn't play well with Windows Defender since it turned it off repeatedly. I finally uninstalled Windows Defender thinking it might allow MSE to perform better. Not so, I now noticed that when booting up, MSE hogged the stage and wouldn't allow my Acronis True Image program to complete its work. Normally TI runs in less than four minutes. I waited over twenty minutes and still no image created. During that period of time, my system was very unresponsive and aggravating to use. I tried to open MSE and see what it claimed to be doing. It wouldn't allow it.

I opened Task Manager and it became obvious what was hogging the processor time and had it pinned on 100%. The memory usage was also high for the service it used.
To add insult to injury, it turned my automatic updates back on without advising me.

In case you think the updates were probably causing all my problems, think again. I checked and eliminated that possibility early on.
I uninstalled MSE yesterday and it's like getting a new computer. MSE won't be going on any system I maintain unless it has super speed and memory. None do yet.
I don't mean to slate MSE unduly, perhaps it works well on some computers but it certainly isn't for mine.
For me, the best AV protection is sitting in front of the monitor. Operate wisely and keep a full year's worth of backup images.

YMMV
@ synysterblk, Dial-A-Fix is a very useful tool but it won't make MSE work any better. I sometimes use DAF for fixing obscure problems that are otherwise beyond understanding. It has proven very useful in those situations and has never harmed any system that I have used it on.