Finding correct trim: Trim up too far and the boat will "porpoise." The nose will be bouncing up and down. From there trim down slowly until its steady. done.
Now you can deal with that issue every time.
I'm assuming that is not the issue because changing prop shouldn't make a huge difference in the trim angle. Maybe a little... not much.
The loss of more than half speed points to either loss of power or spinning the prop on the hub. How did the engine RPM change?
(assuming you didn't have a power head failure)
Too much prop will cause loss of rpm and if you can't get to the correct rpm you can't get in the engine's "power band" (RPM range for best HP)
Prop spinning on hub, the rpm will go up vs normal.
