Determine AVR Speed in software https://forum.arduino.cc/index.php?topic=345862.msg2384213#msg2384213

sketch.ino

#include <avr/sleep.h>
#include <avr/wdt.h>

volatile bool wdtFired;

// watchdog interrupt
ISR (WDT_vect)
{
   wdt_disable();  // disable watchdog
   wdtFired = true;
}  // end of WDT_vect
 
void setup ()
{
  noInterrupts ();           // timed sequence follows
  MCUSR = 0;     
  // allow changes, disable reset
  WDTCSR = bit (WDCE) | bit (WDE);
  // set interrupt mode and an interval
  WDTCSR = bit (WDIE);    // set WDIE, and 16 ms seconds delay
  wdt_reset();  // pat the dog
  wdtFired = false;
  interrupts ();             

  unsigned long startTime = micros ();
  while (!wdtFired)
    { }  // wait for watchdog
  unsigned long endTime = micros ();

  Serial.begin (115200);
  Serial.println ();
  Serial.print (F("Time taken = "));
  Serial.println (endTime - startTime);
}

void loop ()  { }