Obviously, DL makes these decisions based upon all sorts of data inputs, but that much of a gap between other airlines' average block time versus DL's has to have a notable downside cost impact (both in terms of airframe utilization and crew time) versus the industry average. If I could find a source with exportable data, I'd be curious to poke around in Excel to compare DL's block times on a given route versus other airlines - it'd be interesting to do a quick empirical analysis to see if DL is, in fact, more conservative with their scheduling.

Most people are probably quite happy when their flight lands "early," but at some point, it becomes silly in terms of amassing extra costs if you're routinely arriving 30-40 mins ahead of schedule. Perhaps that's their rationale to maintain a superior OTP?