Measuring time between two received bytes on UART in debug script

I can see how to receive bytes from the simulated MCU in a script, but I only see "seconds" variable for execution time, which is not good for me. Is it possible to measure milliseconds?

Using SIGNAL might be a solution, but I worry that running a signal function every 1ms (to update a global variable used in the uart rx handler to check the byte gap timing) will be too resource heavy on the PC.