I've always enabled the resourcepack since starting to use cullleaves (assumed it was required or else not something that would affect FPS, as it was labeled smart), but today I tried without it and I had more FPS consistently when looking at leaves. Then I found why this is the case - the mod still works without the resourcepack and makes more of the insides of leave blocks transparent, which allows for more FPS, and the resourcepack is basically a "fancy" mode even if it's called smart.
This should be noted down somewhere or handled better (e.g a fast fancy switch with tooltip explanation in mod settings?) so that people don't make the same mistake.
Forgive me if OF's smart leaves behave the same way, but I think OF had clear tooltip explanations like Fast: Fastest, Smart: Slower, Fancy: Slowest - so I think CullLeaves can still be improved to confuse users less.