// Print_at_9600.ino
const uint32_t BAUD_RATE = 9600;
unsigned long loopCounter = 0;
unsigned long startTime = 0;
void setup() {
  Serial.begin(BAUD_RATE);
  for (int i = 10; i > 0; i--) {
    Serial.print(' '); Serial.print(i);
    delay(500);
  }
  Serial.println();
  Serial.print("BAUD_RATE = "); Serial.println(BAUD_RATE);
}

void loop() {
  if (loopCounter == 0) {
    startTime = millis();
  }
  loopCounter = loopCounter + 1; // or loopCounter++; i.e. add one each loop
  Serial.println(" a very looooooooooooog msg with looooooooooooots of data");
  Serial.print("Loop:"); Serial.print(loopCounter);
  Serial.print(" mS:");
  Serial.println(millis() - startTime);

  // rest of loop
  delay(1); // assume the rest of your sketch code takes 1mS to execute
}