The 115200bps is equivalent to 115200 pulses /sec ,
the transistor can switch much faster than that.
About the mosfet circuit i think it can work but it is close (it takes about 5us to turn off with 10k),
with 1k resistors in the input it would work 10 times faster (switch off current is controlled with this resistor),
the turn on delay is lower (faster) and depends on the circuit driving it.