I know, but i think the cpu should be enough to do all the work. In normal mc, it does and it works fine. In tekkit, i don't think it does.
It seems that tekkit makes the game alot more intensive, and i don't completely see why. A little bit maybe, but 12 times as intensive?
I also noticed that the framerate is better when you (for example) put yourself in a cobblestone box, or look at the ground (about 60 fps). As soon as you look out into the distance, you can see a great drop in fps (which is not available in normal mc). How can something that does not change any rendering or graphics setting make this happen? The tekkit stuff mostly only adds blocks, items and specific behaviour. The core is the same.
Besides, any calculations that the gpu makes can also be made by the normal cpu, but just less efficient. The best would be if optifine detects how good your graphics card is, and then chooses to calculate with cpu, gpu or both.