Will serial communication have some effect on those critical interrupts?
Only if the serial communication routine itself is written to disable interrupts.
What is the best thing to do. 115200 or 300 baud (extremes).
Assuming that the serial communication routine does not disable the interrupts (see above) then it is the interrupts that will be affecting the serial routine. In that case it seems to me that any interrupts will affect the lower baud rates less than the higher ones.
I guess serial communication is purely hardware related, so no impact on interrupts.
There's some software involved to get the data to the serial communication subsystem but the actual data transmission is done in hardware.
But I have to sure.
You will have to look into the serial communication routines to be sure.
Don