It seems to be lagging in the viewport, and I don't know why. I just downloaded 2.79, since 2.78 seemed to have the same issue. I do not believe earlier versions had this issue beyond that.
If I use the Paint mode in the UV Editor, it doesn't lag at all. How can I fix this?
EDIT: It's been a long time, but I had a PC with 16GB of RAM and a 970. (I now have a much better PC.)
For the record, I tried using low poly vs high poly, and the size of the texture didn't seem to matter. (I literally did the same for both versions and it was still nada.)
I haven't tried it on newer versions of Blender, or my new system, but I will be doing so soon. Since this is a high traffic question, I'll update if I have any issues/solutions.