An alternative to the ShiftRegister74HC595 library is the builtin
shiftOut() function that is part of the Arduino IDE/libraries.

``` 
byte leds = 0;
bitSet(leds, i);

/*
 * updateShiftRegister() - This function sets the latchPin to low,
 * then calls the Arduino function 'shiftOut' to shift out contents
 * of variable 'leds' in the shift register before putting the 'latchPin' high again.
 */
void updateShiftRegister() {
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, LSBFIRST, leds);
    digitalWrite(latchPin, HIGH);
}
```

Faster pin reads. See https://forum.arduino.cc/t/digital-read-write-pinmode-fast-execution/89116

// --- PIN MODE: OUTPUT  --- 

  DDRD |= _BV (0); // pinMode (0, OUTPUT);
  DDRD |= _BV (1); // pinMode (1, OUTPUT);
  DDRD |= _BV (2); // pinMode (2, OUTPUT);
  DDRD |= _BV (3); // pinMode (3, OUTPUT);
  DDRD |= _BV (4); // pinMode (4, OUTPUT);
  DDRD |= _BV (5); // pinMode (5, OUTPUT);
  DDRD |= _BV (6); // pinMode (6, OUTPUT);
  DDRD |= _BV (7); // pinMode (7, OUTPUT);
  DDRB |= _BV (0); // pinMode (8, OUTPUT);
  DDRB |= _BV (1); // pinMode (9, OUTPUT);
  DDRB |= _BV (2); // pinMode (10, OUTPUT);
  DDRB |= _BV (3); // pinMode (11, OUTPUT);
  DDRB |= _BV (4); // pinMode (12, OUTPUT);
  DDRB |= _BV (5); // pinMode (13, OUTPUT);
  DDRC |= _BV (0); // pinMode (A0, OUTPUT);
  DDRC |= _BV (1); // pinMode (A1, OUTPUT);
  DDRC |= _BV (2); // pinMode (A2, OUTPUT);
  DDRC |= _BV (3); // pinMode (A3, OUTPUT);
  DDRC |= _BV (4); // pinMode (A4, OUTPUT);
  DDRC |= _BV (5); // pinMode (A5, OUTPUT);

// --- PIN MODE: INPUT  --- 

  DDRD &= ~_BV (0); // pinMode (0, INPUT);
  DDRD &= ~_BV (1); // pinMode (1, INPUT);
  DDRD &= ~_BV (2); // pinMode (2, INPUT);
  DDRD &= ~_BV (3); // pinMode (3, INPUT);
  DDRD &= ~_BV (4); // pinMode (4, INPUT);
  DDRD &= ~_BV (5); // pinMode (5, INPUT);
  DDRD &= ~_BV (6); // pinMode (6, INPUT);
  DDRD &= ~_BV (7); // pinMode (7, INPUT);
  DDRB &= ~_BV (0); // pinMode (8, INPUT);
  DDRB &= ~_BV (1); // pinMode (9, INPUT);
  DDRB &= ~_BV (2); // pinMode (10, INPUT);
  DDRB &= ~_BV (3); // pinMode (11, INPUT);
  DDRB &= ~_BV (4); // pinMode (12, INPUT);
  DDRB &= ~_BV (5); // pinMode (13, INPUT);
  DDRC &= ~_BV (0); // pinMode (A0, INPUT);
  DDRC &= ~_BV (1); // pinMode (A1, INPUT);
  DDRC &= ~_BV (2); // pinMode (A2, INPUT);
  DDRC &= ~_BV (3); // pinMode (A3, INPUT);
  DDRC &= ~_BV (4); // pinMode (A4, INPUT);
  DDRC &= ~_BV (5); // pinMode (A5, INPUT);

// --- DIGITAL WRITE: HIGH  --- 

  PORTD |= _BV (0); // digitalWrite (0, HIGH);
  PORTD |= _BV (1); // digitalWrite (1, HIGH);
  PORTD |= _BV (2); // digitalWrite (2, HIGH);
  PORTD |= _BV (3); // digitalWrite (3, HIGH);
  PORTD |= _BV (4); // digitalWrite (4, HIGH);
  PORTD |= _BV (5); // digitalWrite (5, HIGH);
  PORTD |= _BV (6); // digitalWrite (6, HIGH);
  PORTD |= _BV (7); // digitalWrite (7, HIGH);

  PORTB |= _BV (0); // digitalWrite (8, HIGH);
  PORTB |= _BV (1); // digitalWrite (9, HIGH);
  PORTB |= _BV (2); // digitalWrite (10, HIGH);
  PORTB |= _BV (3); // digitalWrite (11, HIGH);
  PORTB |= _BV (4); // digitalWrite (12, HIGH);
  PORTB |= _BV (5); // digitalWrite (13, HIGH);

  PORTC |= _BV (0); // digitalWrite (A0, HIGH);
  PORTC |= _BV (1); // digitalWrite (A1, HIGH);
  PORTC |= _BV (2); // digitalWrite (A2, HIGH);
  PORTC |= _BV (3); // digitalWrite (A3, HIGH);
  PORTC |= _BV (4); // digitalWrite (A4, HIGH);
  PORTC |= _BV (5); // digitalWrite (A5, HIGH);

// --- DIGITAL WRITE: LOW  --- 

  PORTD &= ~_BV (0); // digitalWrite (0, LOW);
  PORTD &= ~_BV (1); // digitalWrite (1, LOW);
  PORTD &= ~_BV (2); // digitalWrite (2, LOW);
  PORTD &= ~_BV (3); // digitalWrite (3, LOW);
  PORTD &= ~_BV (4); // digitalWrite (4, LOW);
  PORTD &= ~_BV (5); // digitalWrite (5, LOW);
  PORTD &= ~_BV (6); // digitalWrite (6, LOW);
  PORTD &= ~_BV (7); // digitalWrite (7, LOW);

  PORTB &= ~_BV (0); // digitalWrite (8, LOW);
  PORTB &= ~_BV (1); // digitalWrite (9, LOW);
  PORTB &= ~_BV (2); // digitalWrite (10, LOW);
  PORTB &= ~_BV (3); // digitalWrite (11, LOW);
  PORTB &= ~_BV (4); // digitalWrite (12, LOW);
  PORTB &= ~_BV (5); // digitalWrite (13, LOW);
  
  PORTC &= ~_BV (0); // digitalWrite (A0, LOW);
  PORTC &= ~_BV (1); // digitalWrite (A1, LOW);
  PORTC &= ~_BV (2); // digitalWrite (A2, LOW);
  PORTC &= ~_BV (3); // digitalWrite (A3, LOW);
  PORTC &= ~_BV (4); // digitalWrite (A4, LOW);
  PORTC &= ~_BV (5); // digitalWrite (A5, LOW);

// --- DIGITAL READ  --- 

  x = (PIND & _BV (0)) == 0; // digitalRead (0);
  x = (PIND & _BV (1)) == 0; // digitalRead (1);
  x = (PIND & _BV (2)) == 0; // digitalRead (2);
  x = (PIND & _BV (3)) == 0; // digitalRead (3);
  x = (PIND & _BV (4)) == 0; // digitalRead (4);
  x = (PIND & _BV (5)) == 0; // digitalRead (5);
  x = (PIND & _BV (6)) == 0; // digitalRead (6);
  x = (PIND & _BV (7)) == 0; // digitalRead (7);
  x = (PINB & _BV (0)) == 0; // digitalRead (8);
  x = (PINB & _BV (1)) == 0; // digitalRead (9);
  x = (PINB & _BV (2)) == 0; // digitalRead (10);
  x = (PINB & _BV (3)) == 0; // digitalRead (11);
  x = (PINB & _BV (4)) == 0; // digitalRead (12);
  x = (PINB & _BV (5)) == 0; // digitalRead (13);
  x = (PINC & _BV (0)) == 0; // digitalRead (A0);
  x = (PINC & _BV (1)) == 0; // digitalRead (A1);
  x = (PINC & _BV (2)) == 0; // digitalRead (A2);
  x = (PINC & _BV (3)) == 0; // digitalRead (A3);
  x = (PINC & _BV (4)) == 0; // digitalRead (A4);
  x = (PINC & _BV (5)) == 0; // digitalRead (A5);