Hard running in, definitely.
Fresh bottom end on my MX-5 race car (freshly honed bores, new rings, new bearings, oil pump, etc), pushed it onto the trailer, pushed it onto the dyno, warmed it up, was on the limiter within 5 mins. Great compression, didn't use a single drop of oil all season (previous engine did about 100ml a race).
As Ed says, that first few minutes is crucial, you need pressure from one side (high throttle openings) and vacuum (over-run from high rpm) from the other to seat the rings properly. And you need 'not good' oil (mineral) with the right additives (specific running in oil) to help with this.