Coincidentally, I had a bit of an "experiment" with my BRe and Honda 6hp over the past couple of days.
Not presenting this as scientific in any way...
Simply put, I found I was hard pushed to break the harbour 6 knot speed limit, even at full throttle and one person onboard. As measured by GPS, in counter directions up and down tide.
As expected, appx half throttle was producing about 4.5 mph, full throttle about 6, max.
Big tides at the moment, and the boat's bottom is not clean.
I would suggest that the above might be indicative of hull speed limitations?
I have a sail drive prop, and it seems plenty of poke left to account for loading and associated increased displacement?
I'd also suggest that the answer to your question is that a reasonable cruising speed is about 4.5/5 knots?
Added.
It would be the noise that would initially limit my speed, followed by the fuel consumption which would likely increase exponentially with speed, and so become an issue at over 5 mph?
I didn't perceive that wake would be a problem in our river, but that may just be that we have been so overrun with thoughtless motorboating "Covidimakers" that large wakes now seem to be normal!