All of the above for problem solving. Plus one thing no one has mentioned. Are the throttle plates fully opening when the throttle lever is fully pushed forwards? It's easy to check!
I doubt that you will get more than about 40/42 knotts out of that package with a lightly loaded boat. Once you in the recommended WOT range it's More important to look at revs at verses cruising speed during set-up/prop selection.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.