Just my personal experience:

I've got a super lemon haze going, and from seedling I tossed her straight into a 3 gallon pot, it's been 2 weeks and the 2nd node is barely even coming through - This is indeed because the seedlings roots are expanding - as soon as I see verticle growth, it will be very rapid, and it will only be about a week before I toss her into flower after she starts coming up hard - I'm not worried about the lack of growth because the seedling is still very healthy.

Don't use your food AT ALL until you start seeing vertical movement, the roots are like fine hairs, and if you suffocate them with food, they won't preform the way they're intended to form