Thanks all for your help.
My incoming messages cannot have more than a fixed length and I managed to measure the ISR execution time in microsec for the worst case scenario using a oscilloscope.
Right after entering the ISR i am setting one digital pin to 1 and 1 step before leaving the ISR I am clearing that digital pin.
Max width of that ISR pulse was 16 microsecs which is more than a acceptable ISR length for my case as my serial port is communicating at 57600 bps which means 1 byte every 174 microsec.
Thanks