This is not perfect test but increased the speed with Arduino Due programming port 57600-->115200 --> 230400 and errors started to appear
with 230400 bits/s,
but I do not know about Native port , though , since have not succeeded to install the driver yet, somebody else might know about this ?
with Arduino Mega I could use 230400 and even 460800 but with 460800 some errors started to appear but momentarily,
but like I said these are unofficial tests , since the result depends also on the code of the serial communication and
how often you do Serial.flush();
BR,
Seppo