To be honest, not sure on time departure matters all that much. It's on time arrival that matters more.

All airlines pad their block times quite a bit (fair enough) to manage customers' expectations but more important to address airport and their own operational realities, and quite often, planes would rather sit at the gate instead of wasting gas to:
- Circle / stack if they know they will arrive ahead of schedule (for example, if there is a curfew at the arrival airport -- I once sat at SIN for 35 minutes because tailwinds were so good that we would have circled Switzerland 20 times before ZRH's night curfew would be over) or
- Taxi, waiting for a departure / arrival slot or gate.