To go back to Windows Defender if changing the following and a Speccy reading doesn't show much.
Do at least item 1 and reboot:
1. https://www.howtogeek.com/224981/how-to-stop-windows-10-from-uploading-updates-to-other-pcs-over-the-internet/
2. Go back to Windows Defender and see if the issue fades. Most AV suites today impose quite the extra load. I have to do this step to figure out if it's the AV suite.
3. Reboot. And 15 seconds is NOT EXCESSIVE as more startup checks do occur. I do not consider this to be an issue.
4. Task Manager. When it slows, what is going on?
5. Is there ample free space on all drives? Let's say 100GB?
My PC runs fine for two weeks following the monthly Microsoft update (2nd Tuesday of the month). Then it starts to slow down and becomes almost unusable the week (or so) prior to the next MS update. The symptoms: switching tasks, initiating new tasks, refreshing a browser screen, saving a file, shutting down, opening a new browser tab (Chrome and Edge), searching a folder, copy/paste, even the start-up, etc. may take 15 seconds during week three and 1-2 or more minutes during the week immediately prior to the update to initiate or complete a new task and the blue circle spins and spins. It takes a long time to display pictures as well. There are occasional warnings about a program “Not Responding” (even Windows) or waiting for cache but I wait it out. I’ve run CCleaner, Glary Utilities, Malwarebytes, cleared cache & cookies, etc. without finding anything that can cure the problem and prevent its reoccurrence. It’s been behaving this way for 8+ months. I can’t think of anything that was newly installed at that time. I’m using an three year old Asus (UX303LB) 2.4GHz notebook, 12MB memory (maximum), SSD (drive C is 190GB, is 70%-75% full, no need to defrag a SSD), Internet running at 86Mbs/7.2Mbps per Speedtest.net. Using Avast anti-virus, Google back-up & sync. All software (Windows 10, 64 bit) and patches are current, removed all but a few programs at start-up. The problem is the same whether I’m running one or 10 programs. I really don’t want to re-install Windows! For the first two weeks after the MS update, it’s a really fast laptop PC… then it starts to slooooow down. It’s has to be software but what’s causing it and how do I cure it?

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