In nRF5_SDK_15.0.0_a53641a code, there's such definition:
#define UART_DEFAULT_CONFIG_BAUDRATE 30801920
Which means about 30.8 Mbps.
But in page 345 of nRF52832's Product Specification V1.4, you can see the maxim speed of UARTE32 is 1 Mbps.
This don't make sense, What's the matter?
I got it, by (nrf_uart_baudrate_t)NRF_LOG_BACKEND_UART_BAUDRATE, it's limited to 1 Mbps.
30801920, that's a good value, don't have to be changed for decades.