AVR-Chat

If you mean that served instantly means the latency can't exceed X microseconds, what is X?

The ISR for UART RX should be more than a few 10's of microseconds. If you need a latency of 10 microseconds or less, generally, perhaps you have the wrong design approach.

Well, I suppose that the maximum latency would the the time between 
data coming in on serial line.  That will depend on the highest speed 
you ever plan on using.

Baud rate is about the same bits ber second.  So 9600 baud is about 
100uS per bit.  About 1000uS per byte

115200 baud is about 9 uS per bit, or about 90uS per byte.

That is if I did my math right.  

-Tony