This is likely a cause of Your firewall conflicting with your network proxy.They way firewall is configured (factory settings in mind) is that it places stricter protocol on network traffic.When combined with a proxy, that takes your Ipv6 and sends it to a server bank, then to the target domain.
When a proxy(s) become overloaded, it causes a massive slowdown in traffic. 8.8.8.8 is Google's standard dock address, and is slow( at least when I use it).The proxy you're using is likely the cause for DNS timeout. I would advise doing the following:
1.Check to see if your firewall is blocking a port
sudo telnet google.com 80
2.If you recieve an error, simply go to the activities page to launch the firewall gui and unblock port 80.I would personally disable the proxy via wifi settings or ethernet
3.If apt-get update is still not working, it may be due to the software repositories.If you are tuned to recieved pre-releases, then you may have to remove those pre-release packages.
sudo apt-remove [name of repository]
Then,go to Software and Updates on Activities, uncheck Pre-Release.
I am New to answering and interacting on this site, so I sincerely apologize for any editing mistakes.
I hope this helps, best of luck.
