I feel that a lot of people apply on a whim for rescues after I have started working with a boxer rescue
That is so true. I think only about 25% of our applicants end up adopting. The percentage may be even lower. When we get an application our application committee responds within 24 hours (usually with some follow-up questions and stuff like that). Then we have a volunteer who does a veterinary reference check. That usually takes a couple of days and involves one or more long distance calls. If all goes well, the application is processed fairly quickly. But even then, many applicants have already gone out and bought a puppy out of the paper. Or, after they thought about it more, they realized that they weren't willing to drive two hours to meet a dog. Stuff like that.
Plus, if someone sees a cute dog on our website and THEN applies, yeah, the dog is probably going to be adopted by the time their application is processed (particularly if the dog is young and doesn't have any real "issues").
It does sound like some rescues are overly slow in responding to people, though. Just my personal opinion but I don't think it's too time-consuming just to send at least a quick response to let someone know that their application was received and to thank them for their interest in Boxer Rescue.