LO suck until bottle teat flat

Hi mummies, do u encounter the same problem when feeding LO? 4 weeks ago, it is a chore to bottle feed my LO as he will smile and not drink or wriggle and keep kicking..2 weeks later all the feeding sessions suddenly turned smooth (takes 10mins to feed)..I am using same bottles and teats..now the feeding sessions take 20mins and my LO will suck the teat until it becomes flat. My MIL said flow is too slow (however my LO occasionally still chokes and cough) and she cuts a slit on the teat using scissor. However I googled and saw that when teat becomes flat it is because there is no air in the bottle or the screw cap is too tight. I do not want to change to a faster flow teat as I am trying to bf my baby back..he is now 3mo and using 0m+ teat..any advice based on your experience? TIA!