I've seen the word stupid more times in that post than I've heard this month - once is enough, exaggeration doesn't help
If you've been keeping it on all the time for days on end, I would suggest shutting it down every 2-3 days to let it rest
Get rid of Avira and see what happens - windows defender is quite good or you can go with the free Kaspersky Antivirus
Those 3 times that it slowed down, perhaps there was an update installing? Try changing the timings for update installation to later hours
Try to make sure not everything is updating at once - like the AV, desktop programs, store apps etc - that can also bog down the processor and internet speed