Sorry, you are absolutely correct Windows Defender is native. Sometimes I forget which forum I'm working in. Thanks for straightening me out on that one.
Frankly I'm running out of suggestions, except some information regarding your router. Check to see if there is a possible firmware update that may address the issue.
Try running it briefly without security (open) no encryption to check if there is potential overhead issues or other problems relating to the encryption you've chosen.
Select another channel at the extremes of what might be available Channel three (3) or eleven (11)
Disable IPv6 in the properties of your wireless network adapter.
Try safemode with networking by tapping the F8 key during POST
You never commented on...