I've run into exactly the same issue starting when I got home from work last night. It was running fine when I went to work, my wife played on the server all night with no issues, and when I get home, I'm lucky if i can get 1 FPS. I tried everything. Uninstalled/reinstalled the client, system restore, driver updates and everything. It isn't the server cuz as op stated, happens in single player too. However, my wife doesn't have the issue at all and we run on the same network which is Fiber Optic. My computer is slightly better than hers and I regularly run at 60-100 FPS under the worst of conditions. I use the Curse/Twitch client as I can't seem to get it to download from the FTB client but I've played it from Curse since the beginning with no issue. Beginning to get really frustrated.