I play Minecraft with LabyMod (Optifine) and have only 40 FPS. If I make a shader, I still have 40 FPS, although it would have to sink, why can that be?
Start Minecraft - Go to options → Graphics settings → max. Frame rate = your fps
I have to infinity
Funny, it's max. 60 fps if I set it to maximum
Your PC is just too bad.
No, I have a good PC
Do you have any gigantic texturepack in it?
Do you have a monitor maybe only 40hz or something? Try to make V sync.