If they are severely root bound don't be afraid to rough up the roots a bit. I grab them from the bottom and rip the root ball in half or even quarters about 1/4 - 1/3 of the way up. Then I'll take my hands and rough up, tear, pull the roots that are in the shape of the pot so they spread out a bit.
They are a lot tougher then you may think, I've even taken and ripped up (Pulled out like a weed) some of the second seedlings in a pot, planted them in their own pot, and they do fine. This year I have a second purple piquin because of this technique, and it is only a little bit behind the one that wasn't treated like this.