A diaper should be changed when the baby or toddler has wet or pooped the diaper.
If the baby is just slightly wet and the diaper is not too full and the baby is hungry then feed the baby first and then after you feed them you can change the diaper.
If the baby has a full wet diaper or the baby has a pooped in diaper then change the diaper first and then feed them.
It should not take very long to change the diaper before feeding them.
I always fed the baby after changing them unless they were really wet or had a poop diaper and if that were the case then they would get changed first and then I would feed them.
If the baby is sleeping fine and they pooped or they are wet but not soaked then you can let them sleep and change them when they wake up.