Because probably your videocards don't support "HDR" - effect and that's why you will get low fps.I am saying again: this isn't a lag,neither a problem coming from the server.If you don't want that to happen,do the following on the picture below:
And please to activate in the console "net_graph 3" and to check on "ping","loss" and "choke" and if "loss" is 5,even this is much,TO NOT acuse the server for lagging and to call and consult your internet provider.
And if you videocard is really old,change the resolution to the lowest and you are done.Restart of the game isn't requiered,but just to be sure,you better do it
