Can we have some more information?
What mod pack are you using?
How long have you been playing this world?
Is it just that world or any world?
Do you play SMP or SSP?
Do you have OptiFine installed?
Putting details to fast not fancy does a lot. As does reducing view distance to short.
TBH, there is no magic bullet that will solve your FPS problems aside from dropping a couple hundred dollars on some new hardware.
And it's likely that new hardware won't even help at all.
Lag is unavoidable past a certain point. You have to deal with it. But until then, keep trying things to help your lag issues.
No OptiFine
If he is getting such a low FPS then his problem is very much his hardware. Thus going to new hardware will make quite a difference.And it's likely that new hardware won't even help at all.
Putting details to fast not fancy does a lot. As does reducing view distance to short.
If your PC/laptop possess sufficient amount of RAM, then how about RamDisk?
one thing people keep missing; do you have a big lag machine in the world?
as in a complicated automation that would eat up the CPU
a backed up RP tube line, a overstuffed mob farm, a ton of golems running around will all eat up the CPU
Normal render distance is actually best for FPS. Short and Far are more CPU intensive.