A ton of the mods in FTB are very poorly coded *(eg. to check if solar-panels can see sun, they use the dumbest algorithm possible: a `for` loop that checks every y-value above it for blocks **every freakin' server tick**)*. To test if this is your issue, try going far away from your main-base to someplace you've never been. If the "lag" stops, then the problem is most likely a CPU-hungry mod. There's not much you can do about CPU-hungry mods except turn them off, or *(in single-player only)* just promise yourself not to use blocks from the laggy-mods. Because it's a CPU-issue, reducing the graphics won't help. You could try increasing the process priority, but that likely won't help much, and could cause stability problems. Unfortunately, there's no ultimate-list of laggy mods that I know of, and the list changes with every version anyways. Check the FTB forums, and see which mods most server-administrators are currently disabling, and why.