Yes, it would bother me. It is unprofessional. A therapist is supposed to take care of her own needs outside of your session. Besides, you are right, that time is for you. It's not her lunch break, it's your therapy together. I couldn't take seriously a therapist who ate lunch during my session with them. It would look like they aren't taking themselves and their work with me seriously. Besides, a private practice really can't be that busy, and it's the therapist's responsibility to accept only as many clients as they can handle.