1990 javeli 396 xfd setup problems!

I have a 1990 Javelin 20' with a 225 Evinrude on it. Top speed 40.3 mph
Not sure why. I have a 6" set back jack plate and the cavitation plate is 1.5" up from bottom of boat. I have a 23 pitch cupped viper prop. Any help here would be nice. The 23 pitch runs at 5200 rpms and a 21 runs at 6400??????

First things first.. I'd say, if you haven't already done so, take that motor in and make sure it is running properly. Go from there.

Yep I would take it end and have the motor check out ...that 225 should be able to spin a 25 to 26 pitch up to 5900 to 6000 rpms