1) When using a software UART you can do NOTHING else for that time.
2) Bit time at 115k is ~8usec and it will be difficult to reach that with any precision with an istruction cycle of ~1usec (every path (bit=y, bit=0, start bit, byte complete) need to have the EXACT same number of instruction cycles) you will not have the luxury of padding paths with NOPs.

Erik