Cull Less Leaves skips rendering of inner leaf layers, improving performance.
In a worst-case scenario, overlooking a mangrove swamp, this mod can have up to a 30% performance increase (measured on low-end hardware).
Instead of leaving just the outer layer of leaves, Cull Less Leaves also renders a certain amount of layers defined in the config.
Why not just use Cull Leaves?
Cull Leaves only renders the outermost layer of leaves, which can look quite ugly (see fast culling in image below), but Cull Leaves improves performance more than Cull Less Leaves.
Sodium Configuration