The camera does not delay sending pics. If it tries a couple or three times to send a pic, it quits and it won’t ever send that pic again.
If there are delays, it’s caused by the email server. The pics go from camera to Spartan server, then from Spartan server to your email (or phone text) server.
Do you have the camera set to take more than one pic in a burst? If so, that’s why you are missing pics. Your pic numbers will be every other number or every third number, depending on whether you have 2 or 3 shots set. That’s the first thing I’d check. I run my cameras with only one shot. If you have multiples, it will send the last. (This is for the ATTb, not the xb or A4G).