They may have been slightly root bound just before leaving the cups. Clones and small seedlings tend to only get so big in cups due to the roots filling the cups and it almost stops them in their tracks as far as top growth.. after too long they will also start to show what seems like nutrient issues.. but really it’s their roots aren’t able to take in anything since they are all coiled up at the bottom of the cup.
They look healthy to me, and if at 3 weeks they should be taking off very soon.
Also consider lumens per square foot .