STM32L4R5 + EC25 modem more than 115200


I have build ARMmbed/mbed-os-example-cellular and it is working fine for 115200 baud. It is configured to use PPP. I have tested download speed and it is about 10 kB/s and it is to low for us.
I tried 230400 baud. It is working, but transfer is choppy and results in nearly the same speed.
In case >230400 baud uC just stuck when first response from modem occurs. I suspect to much interrupts occurs.

I reviewed a bit mbed-os STM32L4 uart_driver and it is interrupt driven. Does optimization of this driver will help any way? Do you have any other suggestion?

Br, Matjaz