As mentioned above, it's the terrain that can really slow you down. In my experience the OMM dishes up the most brutal terrain; not the biggest climbs, but rough, slow going and tough nav (especially on the score classes). For Long Score on Dartmoor in 2010 (which wasn't that slow going) 7k/h would've won, 6k/h you'd be top 10 or thereabouts (assuming you made the most of the distance covered).