I'd say that about 2% of my seedlings start to increase when they're around pencil-sized at the leaf bases. I've never really tagged them for follow-through to see if they tend to become "weedy" after they've matured. Otherwise, daylilies tend to remain solitary until they produce their first scape. Fast increase is nice until you find yourself digging and dividing a lot of clumps every year. Good for sales, not so good in the long run. I've noticed that the old-line reds seemed to be some of the most rhizomatous daylilies.
With regard to clumps and self-competition, something that happens in the older, larger clumps is that the mass of roots and fans becomes so thick and tight that the soil directly below the clump becomes very dry, seemingly regardless of how much I water. This tends to force the roots to grow near the surface where the soil dries out quicker. It also encourages mole tunneling, and maybe nesting. In heavier soils, the older fans in a clump will tend to push up to the point where the crowns are exposed, which means that they can't really produce new roots.
