A few options.
- In the launcher you may set aside more RAM for FTB. This is assuming you are running Java 64 bit. 32 bit is limited to 1 GB.
- Installing Optifine may help. In my case it made things worse. It could've been the connected textures effect (where glass appears to be one solid piece, instead of individual blocks), but I am not too worried.
- In your video options, play around with all the options and see what helps. Setting your textures to fast, reducing particles, etc. should help. (i.e. Everything should be logical. Sacrificing quality for speed shouldn't be a difficult concept to grasp I hope.)
What I must say now though is that if you are experiencing a high amount of lag currently, having just started, you may run into larger problems regarding lag/performance issues. FTB, though seemingly low on the graphics scale, is actually quite intensive on you computer. Both CPU (processing) wise, GPU (graphics) wise and RAM (memory) wise. If your computer is a combination of (a) not a gaming computer and (b) more than three years old, you may have significant issues trying to run something like FTB.