I don't know your boat and your motor, but 22mph seems slow even with a stock 10 1/2" prop. Perhaps it was a little damaged when the speed was tested?
Without a TACH, you're shooting blind.
That said, on the boats I've messed with, all aluminum flat bottom and slight semi-V duck/jon boats, the factory 9 7/8 x 11 1/4" prop works extremely well on the F25's. I use one on mine. I bought an expensive Turbo, then had some cupping put into it and some other work done to it, and guess what? It performs exactly the same as the factory $130 aluminum Yamaha 9 7/8 x 11 1/4" prop. The 11 1/4" is a stock 2 stroke prop. The holeshot will suffer just a shade but most don't notice it. I notice it more when I'm loaded heavy. 2 of us in the boat, the big dog (90 lbs), tackle, 3 bags of ice, ice chest full of fish, 15 lb anchor, trolling motor, battery, and all her junk (phone, iPad, drinks, etc) That prop works well on mine. I've seen 6180 RPM last weekend in good heavy cool air, real close to 30 MPH on my rig. Most of the time between 28 and 29 favoring the 28 and around 5900 to 6000 RPM which is right at the top of the maximum for that motor. I've been wanting to try a 12" but haven't found enough left over at the end of the month to buy one (and the dealer won't let me "try" one, unfortunately).
putting a tach on it is simple. See the post: https://www.tinboats.net/forum/viewtopic.php?f=4&t=4391&p=315867&hilit=tach+bracket#p315867