I am truly desperate right now, I've got no idea what to do next and I need help. Recently I replaced my old and bad GPU (Nvidia Geforce 260gtx) with a newer but still not good GPU (AMD Radeon R7 240).
The new one is actually slightly worse in a few aspects, but not a lot, I just got it because it had way more VRAM and new shaders.
After replacing the graphics cards though, playing modded minecraft was impossible because the FPS came to 0, as opposed to about 25-30 with the old card. I opened up the task manager, and saw that the gpu was only working at ~26% 3D load. When playing vanilla minecraft it works at close to 100% and gives me an FPS of about 210.
So at this point I tried to figure out what mod was causing this, so I started removing half the mods except core mods, to try to slowly work my way down to the problematic mod, but it didn't work. It still lagged no matter which half of the mods I used. So now I tried just putting in a few mods: Botania, AE2, Buildcraft, Actually Aditions and Bibliocraft. The FPS was now at about 7, and the GPU was at about 35% load. I then replaced these mods with the rest of my mods, and it went to 0-1 FPS and 25-30% GPU load.
I think this means that the problem isn't a mod, or not one mod anyway. The reason the low FPS happened is the GPU change, but why does it only use 25% of this GPU, and why only when the game is modded? Can you help me make the GPU use it's full potential, or is this even a GPU problem?