Livesplit tends to only lag when your CPU is under heavy load, GPU has little to do with it. I don't know what your CPU is like but check out the load on it when Livesplit starts to lag and see if it's high.

You can also try out WSplit. Although it's less customizable than Livesplit, it's less CPU intensive in my personal experience.

Glad you figured it out.

If you don't mind me asking, what was causing it?