Hmm, I have a 2 week old plant (LR2) that looks like it's on its way to being a similar size as yours by week 3. I thought that because I'm only using a 2700K bulb that possibly this was the reason for mine growing so slowly but this doesn't explain why yours are going so slow.... possibly they'll continue to grow through some of their flowering period?
I'm not sure if that is good or bad since it's taking budding energy away from buds

I noticed you mentioned heat issues, this is something I've also had to deal with. The box has been reaching 31c and I'm sure sometimes it's been hotter than that from time to time but luckily the night temps are dropping below 0c now so that'll be helping since my vents go outside.

All the best

Oscar Mike