Whenever I log in into my account on my 5K 27" 2017 iMac, my whole network starts to have a huge ping to internet, but locally everything is fine. When I ping google, I get over 2000 ms on every device. And It happens only on my user account. I tried to use other cable or use wifi, but every time It's the same. It started a week ago. I have Mac Os Catalina installed.
Apart from Enabling QOS on your network, I think you’ve shown that your internet egress/ingress can’t accommodate the amount of traffic your Mac asks for. One large source of traffic could be iCloud data being cached locally:
You can dig into everything the Mac is doing by opening Activity Monitor - select network and look at the received and sent packet counts and volume. Over 30 to 300 seconds, you should have a good idea if you can or care to quit programs that are asking for more data than the router can accommodate.
Also - check with your ISP / router support team. There are quality of service and other items that can prevent buffer bloat and allow ping/ICMP traffic to not be impacted by large downloads. You don’t have to change anything on the Mac to keep ping times exactly where you wish - just most people and routers will let any one device take all the network - first come, first serve. You could change that on the network side.