Reprap test code explanation

Hi there reprapers!!
Im quite new to this and trying to figure something out.

If I run the test code (Below) with just one motor plugged in to x, my motor goes backwards and forwards in a loop as expected. Im wondering though what sets the speed at which it turns? I can't see any code where the speed is set, just HIGH or LOW?? Can someone explain it to me as I need to alter the code so that I can change the speeds (and durations) of my stepper motors!

Thanks
Mr smith
P.s. code:
#define X_STEP_PIN 54
#define X_DIR_PIN 55
#define X_ENABLE_PIN 38
#define X_MIN_PIN 3
#define X_MAX_PIN 2

#define Y_STEP_PIN 60
#define Y_DIR_PIN 61
#define Y_ENABLE_PIN 56
#define Y_MIN_PIN 14
#define Y_MAX_PIN 15

#define Z_STEP_PIN 46
#define Z_DIR_PIN 48
#define Z_ENABLE_PIN 62
#define Z_MIN_PIN 18
#define Z_MAX_PIN 19

#define E_STEP_PIN 26
#define E_DIR_PIN 28
#define E_ENABLE_PIN 24

#define Q_STEP_PIN 36
#define Q_DIR_PIN 34
#define Q_ENABLE_PIN 30

#define SDPOWER -1
#define SDSS 53
#define LED_PIN 13

#define FAN_PIN 9

#define PS_ON_PIN 12
#define KILL_PIN -1

#define HEATER_0_PIN 10
#define HEATER_1_PIN 8
#define TEMP_0_PIN 13 // ANALOG NUMBERING
#define TEMP_1_PIN 14 // ANALOG NUMBERING

void setup() {
pinMode(FAN_PIN , OUTPUT);
pinMode(HEATER_0_PIN , OUTPUT);
pinMode(HEATER_1_PIN , OUTPUT);
pinMode(LED_PIN , OUTPUT);

pinMode(X_STEP_PIN , OUTPUT);
pinMode(X_DIR_PIN , OUTPUT);
pinMode(X_ENABLE_PIN , OUTPUT);

pinMode(Y_STEP_PIN , OUTPUT);
pinMode(Y_DIR_PIN , OUTPUT);
pinMode(Y_ENABLE_PIN , OUTPUT);

pinMode(Z_STEP_PIN , OUTPUT);
pinMode(Z_DIR_PIN , OUTPUT);
pinMode(Z_ENABLE_PIN , OUTPUT);

pinMode(E_STEP_PIN , OUTPUT);
pinMode(E_DIR_PIN , OUTPUT);
pinMode(E_ENABLE_PIN , OUTPUT);

pinMode(Q_STEP_PIN , OUTPUT);
pinMode(Q_DIR_PIN , OUTPUT);
pinMode(Q_ENABLE_PIN , OUTPUT);

digitalWrite(X_ENABLE_PIN , LOW);
digitalWrite(Y_ENABLE_PIN , LOW);
digitalWrite(Z_ENABLE_PIN , LOW);
digitalWrite(E_ENABLE_PIN , LOW);
digitalWrite(Q_ENABLE_PIN , LOW);
}

void loop () {

if (millis() %1000 <500)
digitalWrite(LED_PIN, HIGH);
else
digitalWrite(LED_PIN, LOW);

if (millis() %1000 <300) {
digitalWrite(HEATER_0_PIN, HIGH);
digitalWrite(HEATER_1_PIN, LOW);
digitalWrite(FAN_PIN, LOW);
} else if (millis() %1000 <600) {
digitalWrite(HEATER_0_PIN, LOW);
digitalWrite(HEATER_1_PIN, HIGH);
digitalWrite(FAN_PIN, LOW);
} else {
digitalWrite(HEATER_0_PIN, LOW);
digitalWrite(HEATER_1_PIN, LOW);
digitalWrite(FAN_PIN, HIGH);
}

if (millis() %10000 <5000) {
digitalWrite(X_DIR_PIN , HIGH);
digitalWrite(Y_DIR_PIN , HIGH);
digitalWrite(Z_DIR_PIN , HIGH);
digitalWrite(E_DIR_PIN , HIGH);
digitalWrite(Q_DIR_PIN , HIGH);
}
else {
digitalWrite(X_DIR_PIN , LOW);
digitalWrite(Y_DIR_PIN , LOW);
digitalWrite(Z_DIR_PIN , LOW);
digitalWrite(E_DIR_PIN , LOW);
digitalWrite(Q_DIR_PIN , LOW);
}

digitalWrite(X_STEP_PIN , HIGH);
digitalWrite(Y_STEP_PIN , HIGH);
digitalWrite(Z_STEP_PIN , HIGH);
digitalWrite(E_STEP_PIN , HIGH);
digitalWrite(Q_STEP_PIN , HIGH);
delay(1);

digitalWrite(X_STEP_PIN , LOW);
digitalWrite(Y_STEP_PIN , LOW);
digitalWrite(Z_STEP_PIN , LOW);
digitalWrite(E_STEP_PIN , LOW);
digitalWrite(Q_STEP_PIN , LOW);
}