Question: what could be the cause of the "slowness" of the boat ?

Expectations.

Hull speed is the theoretical limit for any practical amount of power
unless the hull shape is such that planing lift can start to reduce
the displacement. You have far from that amount of power.

The 1.34 figure would also be for a fairly slender (in flow terms, not
necessarily length to beam) hull. The number goes down as the hull
gets fatter. 1.25 is a more typical number for vessels as heavy as
cruising sailboats and heavy power vessels but they will take a lot of
power to get up to it.

If the hull is making the waves and trimming as you describe, cleaning
the bottom and fiddling with the prop probably won't make much of a
difference.

A lot has to do with the flow angles in the run. The bow makes
rebound up and push the stern ahead, recovering some of the energy
expended in making them and helping keep the stern up. Once the angle
between the run and the direction of motion exceeds 12 - 15 degrees,
the flow separates and the space between the smooth flow and the hull
fills with lazy eddies of water that largely move along with the hull.
Energy from the wave train can not be returned to the hull through the
zone. The effective waterline of your hull is actually from the bow
to the point where the flow lines (generally along the diagonals)
exceeds this critical angle.

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Roger Long