About mobile network searching strategy, I did some tests.
I am just reporting, only those I consider most interesting.
SIM1: Iliad
4Gcl: off
Netw: Auto
GPSi: 7/27, 9000m
VPN : on, Proton free on UDP
CrSr: uninstalled
Batt: >10%Data: Enabled
Mode: prefer 4G
Tine: 6m30s and not foundData: Enabled
Mode: 2G only
Tine: 23s + 10s to data switch 2G → 3G, 4G immediate
Tine: 41s + 5s to data switch 2G → 3G, 4G immediate
Tine: 2m aborted
Tine: 23s + 5s to data switch 2G → 3G, 4G immediate
Tine: 41s + 12s to data switch 2G → 3G, 4G immediateData: Disabled
Mode: 2G only
Tine: 5s but after 3m did not found 3G, yet
Tine: 3s, data on 5s to connect, witch to data 3G in 13s, 4G immediate
Tine: 36s, data on 5s to connect, witch to data 3G in 10s, 4G immediate
Tine: 7s, data on 3s to connect, witch to data 3G in 5s, 4G immediate
It is quite clear that the procedure have a great impact on the performances. However, all these numbers are out of range compared with the SIM in the 2nd slot configured for no-data, no-routing, 2G only just for receiving calls and texts.
The big difference is about restarting the SFOS network applications stack.
Apps -> Settings -> Info:Utilities -> Restart Network Subsystem
The VPN enabled relying on UDP may have its part¹ in this mess but the story would not change so much disabling it completely.
The Restart Network Subsystem is the key-workaround and probably it would be better to have an option for each SIM to do it automatically when the airplane mode is switched off.
Data: Enabled
Mode: prefer 4G
Tine: 6m30s and not found
This worst case can be solved adding the Uitlities to the TopMenu and in that case the workaround procedure would be
Airplane mode off, Utilities → Restart
NOTE
¹ A VPN based on UDP may affect the 4G registration expecially if mobile data are active and the VPN server is under heavy load? Surpringly, this means that the full 4G registration of the device in the mobile network is not all based on out-of-band communication. Perhaps someone extended the standard 4G registration protocol using 3G initially deisgned hardware? 