The push message of the TestFlight version is much slower than that of the App Store version because there is a priority for the APNs server to send messages in the test version?
TestFlight versions and App Store versions are normally identical, both in code sense, and in the sense of being treated for notifications.
If you are seeing a longer delay in receiving high priority (P 10) notifications it is likely coincidental. If you are seeing a longer delay with low priority (P 5) notifications, those could be delayed different durations depending on device state. Or your observations could still be coincidental.