This is a typical problem faced in RS485 based systems. You have to make allowance for the receiver to get a character, buffer it, process it and come back to the point where it is ready to read the next character. I have not worked with a hardware serial port PIC till now, but having experience with the 8051 family, I know that these things matter.

So, what I do is to send a packet to the slave - wait for a few ms for it to reply / handle the packet and then send the next packet if needed.

So, the turnaround time I mentioned is not character to character but packet to packet.

Jerson