Is this single player or on a server? What hardware are you using? Any extra mods such as forge essentials? Is there any particular chunk that it's lagging in? If so what's in that chunk?
Personally I've been getting lots of lag on my 3gb fragnet server for the last couple of days, reverting to an earlier backup seems to stop it, and building something very simple with a couple of timers, 3 transposers, an automatic crafting table mk II, a rotary macerator, and two enderchests starts to hog the cpu and ram until they both reach 100%... after a minute of this the server kicks me off cause it can't respond to my client any more and i either have to revert to an earlier backup where those blocks don't exist or use mcedit to remove them from the world.
I can't believe I'd be the only one having such problems if it's a problem with one of those blocks, and it does seem to be because I can recreate it in a different chunk if I use those components again. My server is rated for 36 people... it's a server I run for a few family members only so there's only a maximum of 5 people on it at any one time so it's not 'normal play' that's causing the lag, it's a bug either in the tekkit lite group of mods or the only other one I run which is Forge Essentials.
I came across your posts as I was searching for similar problems to my own...the point of my post really is to say, without more information about your circumstances I don't know if it is or it isn't - and neither will anyone else.
Edit: I think I may have found references to my own lag problems being something to do with the rotary macerator having a redpower supply turned on when I log off