run 64bit java
spread your base out more
reduce the amount of machines you have and become more efficient
reduce the draw distance to normal or small
set the graphics to fast
install optifine
get more RAM
upgrade the CPU
Also, lag is a slang term used for network latency, not low framerates, but i assume youre talking about framerate. Every FTB pack is going to eat all but med-high to highend CPUs when you start building more things. Id recommend a i5 2500k as you can find them used for cheaper more easily.