Just to chime in I a missed notification yesterday.
v 2.1.2.115 Not that it probably matters
Using Pushover
For at least a week there appears to be a average delay of 5 seconds, sometimes more.
I don't know if the message process first goes through the Hubitat cloud or if it directly goes to pushover but the delay is about the same whether I use pushover or the Hubitat method.
It sounds like a phone manufacturer cloud issue with their notification mechanism then. (APN for apple or FCM for android) You did not mention what type of phone it is, but if you are seeing delays with pushover, that has nothing to do with the Hubitat cloud it goes directly to their servers from your hub.
Notifications always go through clouds, not just one. There is no guarantee that notifications will arrive and in what time they arrive. Even the Search Giant and the Not Banana won’t guarantee it
This is why my hope of an increase in SMS messages happens. I used SMS when I was on SmartThings and the delivery was almost instantaneous. When considering Hubitat as my platform this was a major sticking point. While a little delay is fine for most things there are other situations where speed is utmost important.