Even when not triggering sound on load, Safari has a delay.

When you click a button or trigger a sound in a script, Firefox and Chrome play it instantly and reliably with no delay.

Safari (10.1.2) always has a ~500ms to ~1000ms delay. It's as if the sound is not cached in RAM by Safari. It behaves as if it has to load it from the hard disk cache each time it is requested causing comparatively sluggish behavior.

Note: I also tested it in Safari 11.0 on macOS Sierra with the same results. It also seems to play the sample at full volume once then it turns the volume down for subsequent plays.

This is a very short sound meant to sound like a click or a quick tap.