i have this all the time. I run monster on an old laptop so I have to make serious choices on what I leave running and what I disable, graphics options, etc.
1) render: far = bad. drop that a couple notches. even with tiny distance it still spikes occasionaly, but at least its smooth
2) set all the graphics down to minimal settings. smooth lighting off, particles minimal, no advanced gl, no clouds
3) no sounds. a few machines and items make sounds, and things like the mfr harvester and mob grinders, those flood you with a lot of sounds.
4) try setting task priority higher. assuming windows, launch ftb, open your task manager, find the javaw process thats killing your cpu, right click on it and 'Change Priority', set it to High. do NOT do realtime, your machine will puke steamy chunks in your lap.
5) in the launcher options tab, try giving java some more memory if you can afford it. I cruise with 2.5 gigs and seems to behave
6) anything running thats not mc related? torrents, chat apps, email, etc. memory and bandwidth hogs, that might impact things. i know modern machines should multitask, and do it well, but with MC...you really dont wanna do that
the last option: disable mods. lots of em. Most times I'm doing test builds in creative or survival single player, so I dont need everything running. I focus mostly on tech mods so mystcraft, twighlight forest, thaumcraft and all the little things like obsidiplates, flatsigns, denpipes...they all get disabled. up to you how you juggle the list, as long as you juggle it. trust me, monster with a full mod list is not possible on this laptop
I have tried optifine with the launcher with mixed success..depends on your hardware. It doesnt do much for me, but you can try it and see if it gives you a little edge.