I can’t find the option to “Add an extension”.

You are so wrong about that. ESET, Emisoft or Kaspersky are about the only three paid windows 10 AV’s that are any good. The rest are garbage. I work on 8 to 12 pc’s a day and I will usually just install defender on windows 8 or 10 or microsoft security essentials on windows 7 if the client want’s a free AV.. The rest slow your computer to a crawl… Defender may slow down some pc’s but in my experience it’s mainly Dell that experience this.

Malwarebytes free version doesn’t run in the background but can find problems that have already occurred. Defender and malwarebytes is all anyone needs for security on windows 10. The paid version of malwarebytes is only 39.95 a year. ESET nod 32 is another good AV at 39 a year.. Webroot, avast, norton, macafee, they are all garbage. I am a remote support tech and I always talk my clients into removing these as they cause more problems with performance then they help.

Windows Defender typically has 4 different scheduled scans:
-Windows Defender Cache Maintenance
-Windows Defender Cleanup
-Windows Defender Scheduled Scan
-Windows Defender Verification.

When you get to the Windows Defender tab in Task Scheduler (following the above instructions), If the conditions fields are grayed out, you have to edit each schedule individually. Right click on the task in the top portion of the page [where it says Condition: Ready] and click Properties. There you can change the parameters of the tabs for each type of schedule.

Before following any of the advice in this article, I would allow Defender to complete whatever activity it is performing. I’ve found that running a full scan when I don’t need my computer is the best way to get it to smooth out. If people are interrupting the scans because it’s hogging resources, of course it will attempt to resume until it has completed.

Run a full scan, go to breakfast/lunch/dinner/or movie, THEN change the settings so that it skips already scanned folders that would normally take a long time (like where you have video files). I did this after upgrading to Win 10 and have never had Defender slow down my system. None of the default tasks is set to begin if idle. On the Conditions tab, put a check in the box for “Start the task only if idle for:” The default is 10 minutes, but you can set for 1, 5, 10, 15, or 30 minutes or 1 hour.

Recheck your settings after doing a big Windows Update, because many times the big updates will change some things back to their defaults.

Delete McAfee. It’s proven to slow down your computer. Actually, delete all of those. For basic internet use, all you need is malwarebytes (it’s free) and windows defender.

Admin account is the one by default, so you seems to be on admin account.

How do you get on with an admin account? I believe my system came with Windows 10 Home version and I have to log In with a password but was never offered an admin account. I don’t use a Microsoft account.

were you on admin account?