Jack-Rabbit
I have an issue with internet speed.  With just basic Windows 10 defender, I get practically my full download speed 910/800 Mbps.  If I enable any other antivirus my speed is cut in half.  I've tried AVG, Avast and Bitdefender and the same issue occurs and when they are enabled it reduces my speed to 375 to 450.  Anyone have any thoughts.
 
Hardwired connection
Win 10 Pro 64
32 gig 3200mhz
Ryzen 7 3800x
Asus x570 Tuf

In some cases, antivirus can actually slow your internet. As we mentioned before, antivirus software can use some of your bandwidth to update its virus definitions and software. This will be a lot more noticeable if you’re on a slower bandwidth. Another way that antivirus may slow your internet speed is through built-in firewall settings or a VPN.
It’s a well-known fact that firewalls slow your internet speed (depending on firewall configuration). Thus, when an antivirus is running its firewall on top of a Windows built-in firewall, it can wreak havoc on your internet speed. This is why it is generally suggested to choose one or the other. Imagine a firewall as a traffic checkpoint – all the traffic (your data) slows down to pass the checkpoint. So imagine having two consecutive checkpoints – traffic nearly comes to a standstill.