as odd as it may sound it may simply just be the temp of the rat I had a heck of a time figuring out just the right way to bring the body heat up with out cooking the food in the process when learning to feed f/t and the trick I have found is to thaw it completely at room temperature for at least 2-5 hours depending on size once it is completely thawed to room temp thoroughly then hit it with a blow dryer for like 20 -30 sec then offer it usually the temp is high enough externally to keep them interested and not so high that you start to cook it.
Time well you could always thing of it this way with it pre killed it can sit down to a nice big meal and takes its time consuming every drop cause it doesn't fight back