There can be several reasons as to why there is a delay for notifications when compared to notifications arriving to your old iPhone vs your new iPhone.
- Ensure that your new iPhone is not set to Low Power Mode.
- Ensure that you are comparing both devices while either both are connected to Cellular data (same carrier) or both are connected to the same WiFi network.
- Ensure that neither are connected to VPN.
- Ensure that the new iPhone has been setup 24-48 hours prior to testing.
If all of the above requirements are met for this testing, and it does not help, then try Resetting Network Settings on the new iPhone. Settings ➡️ General ➡️ Transfer or Reset iPhone ➡️ Reset ➡️ Reset Network Settings
⚠️ This erases Saved Wi-Fi passwords and VPN settings, however, it often fixes new-device push registration lag.
Axel F.