I am having a similar issue with my DF70A and I believe it is prop related.
To be fair I do not know the size of my prop yet, but I will check either this evening or tomorrow.
My DF70A doesn't rev past 4500 at 13 knots.
I have been advised to go down in prop.
We'll see...

To perform at their best, and more importantly, to prevent damaging the engine due to excessive lugging, your Suzukis need to be propped to enable them to reach their rated mx rpm (or close to that).

Lugging is excessive load on a engine, in simple terms it is like driving your car around at 20mph in top gear all the time! The engine reacts to load by forcing more fuel into the cylinders and advancing timing, but the excessive load is pushing back against the pistons creating stress, which creates more heat, and to make things worse, the fuel cant be burnt properly and is forced past the rings and dilutes the engine oil which reduces it ability to properly lubricate moving parts. So lugging is bad from all angles you want to look at it from, it will reduce your engine's life without a doubt.

It is completely false to think that you will get faster top speed spinning a big prop that wont let the engine reach max rpm.

So Pontoonguy needs to take the advice and reduce prop pitch.

And Moggy - well, if you can only get 4500rpm at WOT with the engine trimmed out, then you are BADLY lugging your engine and will be significantly reducing its serviceable life. You will also be using 20-30% more fuel than you could be if propped correctly and the acceleration will be awful.

If that truly is your max revs at WOT, then its either one of the worst examples of overpropping I have heard of, or something is not right with your engine. Mahbe its not running on all cylinders, or something else is wong.

You really need to find out what is going on and make sure the engine is running properly and is in good condition, and if it is, then get a prop that will get you close to 6000rpm.

Both of you need to check your engine height. An engine that is set too deep in the water creates extra drag that slows the boat down, destroys fuel economy, and adversely affects the boats attitude and handling. Maybe not so much the attitude of pontoons, but certainly any mono or cat hull's general attitude and handling will be adversely affected.

Please read the sticky thread towards the top of this forum that deals with selecting the right prop and setting engine height correctly for your Suzuki. It tells you a lot of useful info about how important this is and how to get things set up right.