Yeah Optifine varies significantly on it’s different versions, in my experience it significantly harms my fps in many setups.
Are you using Optifine with gt6? How is mc with just forge with and without Optifine?
If Optifine is involved basically all things go out the window, and since your trace was graphical related when gt6 doesn’t really do anything graphical at all then I’m leaning to Optifine interacting badly (yet again).