It's normal that when they first start out they'll eat real slowly - that's coz they are still figuring out how to eat from a spoon and also getting used to the new thing that they're eating. You have to give your baby time to adjust and not try to feed food via bottle. Otherwise he'll take even longer to be accustomed to eating and you can't feed him everything using a bottle right? Plus since he's used to drinking milk very fast, there's a high chance that he'd choke if you feed cereal using bottle. Just hang on in there and let your little one get used to the experience of eating
If you're afraid that the food turns cold, you can scoop only a little bit out at a time and leave the rest in a food warmer or a steamer so that baby gets to eat constantly warm food.