Why does the delay between bytes increase when I increase UART baud rate?
The higher baudrate makes the UART module transmit faster, so you have more "spacing" between the bytes, but it seems to me that the rate at which you're transmitting the bytes is still the same. Your timing diagrams are missing total time so its hard to see if this is the case. Don't compare last edge to first edge. Compare the first edges of the the two bytes. Are these still the same?