So basically if the network is "busy" - something is moving, downloading, uploading - then Max viewport becomes very slow if there are proxies used?
This sounds like there could be a bug with proxies refreshing over network with no reason. I'll move this to bug reporting, and we will have to try reproducing this.
There is also another similar report submitted in our support portal (#7047, also logged in task tracker), but the issue there *may be* caused by the Quadro K5200 GPU, as it does not happen on other computers in the office.