Hi Ruslan,
All my code is running inside the module (Q26 based) so I don’t have any metrics for sending SMS using a modem as a USB device.
I do know that SMS done as a store and forward and has no guaranteed delivery time - the networks do them as best effort and if the network is busy, the SMS is delayed. My personal best effort is 4 days from sending to when the SMS was delivered! After this effort, I stopped using SMS for delivering alerts and make a CSD phone call instead. Maybe your network is congested?
8 seconds sounds pretty good though.
ciao, Dave