Daylily genes

Some of you are experts on Hemerocallis - Chris in LI and I think Auralie knows a thing or two. Anyone have a good answer for this?
I have about 12 named cultivars (though a few have lost their names) of daylilies. All are dormant growers. A few are new so I can't say much about their growth habits, but of the others, most seem to be somewhat slow-growing to me. They might put on a new fan, occasionally two each year. By contrast H. 'Crown Royal' must be divided every third year and then it takes all manner of tools to pry them apart; it's that vigorous. I'll replant 8 again, but I'll have filled 30 pots.
Is it in the plant's genes? What makes one grow so much more vigorously? Though they grow in different spots, I beleive they all get similar, soil, sun, moisture, & fertilizer conditions.
I am familiar with the difference of Stella d'Oro, but none of these daylilies seem to be of that type. 

Does anyone know of a way I can beef up the other hems more quickly?

Thanks


Kitty
neIN, Zone 5 
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