Why Should I Avoid delay()?

delay pauses your entire program. While it waits, your board cannot read inputs or respond to changes.

What delay really does

• Stops the loop from running.
• Misses button presses and sensor events.
• Makes multiple actions feel slow or unresponsive.

What to use instead

Use millis to check time without blocking:

unsigned long last = 0;
void loop() {
  if (millis() - last >= 1000) {
    last = millis();
    // do timed action
  }
}

Related: How do I blink without delay? · How do I do multiple things at once? · How do I make a timer?