SerialUSB doesn't have a speed. It's USB speed.
Compared to the traditional UART, its like lightning speed
Try changing the SerialUSB.begin( to anything you want ), it does not matter. Open it in the Serial monitor, and that baud setting can again be anything, it does not matter.
SerialUSB comes through packets, not bitbanging like traditional UART, or like simulated UART through FTDI like adapters.
Anyway, it will be much much faster than you will ever need.
Your limitation will be the ADC sampling speed, and that depends how it is configured. Right now for OLED display purposes, there is quite a bit of oversampling and averaging done in the ADC itself, before taking a reading.