Just putting this out there to help people that may have the same issue I just resolved on my PC.
After hearing about Dawnguard, I started working on a character that would fit in the more with the whole Carpathian theme more than most of my characters. So I get them to where I wanted and shelve it for awhile. Upon starting to play this specific save again a few months and mods later, I noticed a very annoying hitch in my game every few minutes that would last for about a full second. After lots of processes of elimination that I'll spare you, it turned out that my save file for that character had somehow grown to 99 megs, which was a little alarming since they're normally in the 3-12 range, give or take. After a little more sleuthing, I figured out that the scripting in the "Project Reality: Climates of Skyrim" and "Sounds of Skyrim: Civilization" mods were making the savegame file size skyrocket at a rate of 2mb per minute or so. I removed those two specific mods, then waited in-game for 24 hours so the exterior cells reset, then saved again. After a restart and running around for a few minutes, it went back to normal.
Other people seem to have also had this issue with other mods as well, such as the "Travelers and Adventurers" mod.
Hope that saves some of you the frustration. :D