Why 9 minutes
Nine minutes is the snooze interval Apple inherited from mechanical clock radios, where the gearing could not cleanly produce ten. It survives as a default on hundreds of millions of phones, which makes it the most-used nine-minute interval in human history and almost certainly an accident.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 9 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.