CKKwan Windows Defender is free so what do you expected? Hence, I think you should just disable the scan schedule. FYI, I also don't have such issue you mentioned.

im using windows defender onwin10, with ssd. so far happy with it. yes im monitoring my performance since this is brand new ssd

no issue using window defender here.
SSD user here also.

If there's an update ( background process ) then only it will shoot up because it's writing to the disk.

I had been using Windows 10 built-in defender quite a long period lastly I shifted to paid Internet security.
Many problems occur using Windows defender and firewall.

I find using windows defender not too bad actually~ Doesn't really give me a problem and is free so ok la~ Helps me detect virus and I switch off the schedule scanning so it is fine.

once u install 3rd party insurance the windows defender is deactivated isn't

yes it will run the vendor's

Interesting

WD serve me well all this time. on top of this I use malwarebytes as well.

Wondering if it has anything to do with the size of the disk... My machines all have at least 240gb hdd/ssd for the OS and none of them have this issue.

is it the Hyper V ? most of us don't have any problems installing direct to PC

I am using it since 2016.
No problem.

is this Defender reliable compare to other antivirus?

I prefer Defender since it is part of the OS, so it should be more efficient.

actually i am also thinking to use Defender, it should be at least on par with Kaspersky one