When i am playing with the default texture pack my fps can go up to 700 fps but when i installed the Sphax 128p the fps dropped to 15 fps.
I have 16 gb ram and quadcore i7 3.5 ghz
I run the same texture pack on tekkit clasic but with all the tekkit items (3 times more than vanilla) and on this version my fps is 300 stable fps.
I also had the 128 sphax on the older minecraft versions so this is the first time it happen
I hope you can find the problem and fix it