It is not only a Verizon problem, as I am on T-Mobile...
I have had this problem since my Samsung S10+, and it continued on my S20 Ultra. I do eventually get all of my notifications, and most of them are in a timely manor, but almost always when I unlock the phone I will get a straggler. I also have a wifi controlled garage door opener (Meross) that notifies me of the door open/close that is ALWAYS delayed, anywhere from 10 - 20 minutes (not very helpful). I have gone through every setting numerous times, I have played with sleeping/ not sleeping apps, battery optimization...everything, and nothing changes----until, I read about a command to disable Doze, it's an ADB command "adb shell dumpsys deviceidle disable" and it will respond with "Deep idle mode disabled" and "Light idle mode disabled". The next time I left the house I got an immediate notification from the opener that the door was opened...and when I returned an hour later it again notified me immediately when the door was opened and then closed. This command is not persistent, so it will be disabled on a re-boot, so you will have to re-enter it on each boot (unless someone can figure out how to have it run automatically with tasker or such). I only did this yesterday afternoon, so I don't know how much battery will be lost, but based on yesterday I should still make it through a day with no problem. I'll keep an eye on it and see.
I think that this might prove that it is a phone (Samsung or Android) problem rather than a settings problem (I'm thinking Sammy).