This came as a surprise to me - I then reduced my pressure on my mountain bike and indeed I did go quicker, even uphill. It seems counter intuitive, but it's true.
On a smooth surface the following applies: The higher the tire pressure,
the less is tire deformation and thus rolling resistance.
Off road it is exactly the reverse: The lower the inflation pressure, the
lower the rolling resistance. This applies equally on hard gravel roads
and soft forest tracks. Explanation: A tire with low inflation pressure can
adapt better to a rugged surface. It sinks into the ground less and the
whole rotational mass is retarded much less by the uneven surface.