Modified Reaction Timer Code

I modified reaction timer code so that a piezo sensor would start timer, LED would flash 3 times before random count, then LED would turn on and timer would start counting. When piezo sensor touched again LED would go out and timer would stop. Everything working fine that way, but now I want to add a piezo as a buzzer to sound at the same time LED turns on and goes out when LED turns off, which it does, but
after the first start when I start timer again the LED and the random lines take 3 times as long to run for some reason the process takes longer. What I want to do, is have LED turn on and buzzer sound, but only sound for only 250ms without effecting timer count. I tried different ways but no luck. Can you help with this project? code below.

#include 



unsigned long start, stop;

const int knockSensor = A1;

const int ledPin = 13;

const int piezo = 8;



void setup() {

randomSeed(analogRead(0));

pinMode(ledPin, OUTPUT);



DigitShield.begin();

DigitShield.setPrecision(2);

DigitShield.setBlank(true);

}



void loop() {



while (analogRead(knockSensor) <=10);    // wait until button press

for (int i=0; i<3; i++)                              // declares i, tests if less

{                                                              // than 3, increments i by 1

digitalWrite(13, HIGH);                            // turns pin 13 on

delay(500);                                            // pauses for 1/2 second

digitalWrite(13, LOW);                            // turns pin 13 off

delay(500);                                           // pauses for 1/2 second

}

DigitShield.setBlank(true);                     // turn off the display

DigitShield.setLeadingZeros(true);

delay(random(2000, 5000));                  // delay from 2 to 5 seconds

digitalWrite(ledPin, HIGH);

tone(8, 2000);

start = millis();

DigitShield.setBlank(false);

while (true) {

DigitShield.setValue((unsigned long)(millis() - start)/1000.0);

if (analogRead(knockSensor) >=10) {

stop = millis();

break;

}

}

noTone(8);

digitalWrite(ledPin, LOW);

DigitShield.setValue((unsigned long)(stop-start)/1000.0);

DigitShield.setBlank(false);

delay(1000);

}