Vari-flow or fast flow?

My LO is 5 months and is currently using size 2 on the tommee tippee bottles. She is also on a comfort milk for constipation which is apparently thicker than normal milks. She has been using the size 2 teats for 5weeks now as it was recommended I use them with this milk. For the last week she has been uninterested in eating and I feel it is taking her longer to feed when she does eat. She seems to get bored even sometimes after 1oz! I thought it could be the constipation at first and maybe her stomach was hurting, then I thought it could be teething but I’m still not sure it is that. So I’m going to try changing the teats again. I don’t know whether to use a var-flow or a fast flow, I don’t understand how the vari-flow works tbh and I’ve never seen one.

Does anyone have any experience with them?

Any positives or negatives?