I feel for you. The most I've ever done is eight (I think). I try to limit my shops to five a day whenever possible.
Some suggestions:
Two ways that I see doing these shops under those restrictions:
1 - Take a laptop with you that has internet connectivity and a mobile internet (mobile WiFi). Perform the shop and then do the report. Then move on to the next shop. BTW, if you knew of local WiFi hotspots along your route, you could eliminate the mobile WiFi device.
2 - Start your shops later. Meaning late in the evening performing half before midnight and the nsecond half after midnight.
Like everyone else says, there is just no way these shops take 45 minutes! When it was the other company that had these shops, I was doing them in 10 minutes. Plus, I always had a very high shopper rating.
You indeed, get in a rhythm when doing these and know what to look for and how to do them.
I also believe this Email was directly sent to you and not others. In the past, I've received Emails about not finding "enough" flaws or problems that other shoppers have found at the same location. I know other shoppers have received Emails who have found too many problems at various sites. I'm certain it is a mathematical algorithm in which shoppers are judged.
I also will add that although shoppers are not supposed to talk with the attendants at these sites, I have heard complaints about other shoppers being mean, nasty, demeaning and overly critical. I've even heard that some shoppers have wives/girlfriends, husbands/boyfriends, and/or children accompanying them. That doesn't matter to me. Whatever.
Some employees have said that the other shopper marked them off for not having certain decals on their pumps when clearly the photos of the pumps show otherwise. Hearing this makes me think that some shoppers have found a way to stay within the shopper algorithm.
I just nod, smile and try to converse as little as possible, staying away from the shopper findings.