long lastMillis = 0;

long loops = 0;

​

void setup(){

  Serial.begin(9600);

}

​

void loop(){

  long currentMillis = millis();

  loops++;

  /* By doing complex math, reading sensors, using the "delay" function,

  *  etc you will increase the time required to finish the loop,

  *  which will decrease the number of loops per second.

  */

​

  if(currentMillis - lastMillis > 1000){

    Serial.print("Loops last second:");

    Serial.println(loops);

    lastMillis = currentMillis;

    loops = 0;

  }

}