Speed is not a good measure as resistance curve becomes exponential past plane. Speed won’t change that much with RPM, but engine load and fuel consumption will.
You might just need a higher pitch on your propeller.
Your propeller would be designed to cavitate 5 to 8% at full power, design RPM (4800), so at 5900 you would also be cavitating heavily, which could also cause you not to increase speed.
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