I'd actually experienced this a little bit in 1.7 but I feel like the issue's only gotten worse. Also forgive me if it's an issue already noted, I had trouble finding related help at all that wasn't from 2012.

For some reason only this one area of my game, the map will severely lag and the FPS will start dropping to 3 - 30, from 60, and then continue to taper off back to 60 as I leave the area or just stop moving. In the past this was because of redstone triggers, but I have no redstone in this area, although I have built a lot in it.

I turned off clouds, limited the amount of chunks loaded, did everything graphically I could do but it doesn't seem to affect it.

Other than that everything is fine. It's just in this one area and possibly a couple others I frequent where it's a struggle to get past, and I only make this now because the area is where I frequently play and is much larger than others.