The Run Time Count function on the E7/E8/E8a emulators uses the host PC as the timer resource. Therefore, a time measurement in smaller units than milliseconds is not available. Also, note that the Run Time Count includes both the debugger software processing time and the execution time.
However, the E8a emulator supports a method for high precision time measurement using the internal MCU timer. A detailed explanation is provided in the following application note:
E8a Emulator Outline of the Start/Stop Feature and an Example of Its Application for Time Measurement (REJ06J0068-0100)