Without delving into academic complexities, speed is a function of thrust available vs total drag, and in particular form or parasite drag. Being so aerodynamically clean, Seagull's PC-9 is fast in level flight with a typical sport .46 engine.

The throttle is what will slow you down. Once reduced to flight idle, deceleration is a function of drag. A lower pitch blade will increase drag at flight idle therefore assisting deceleration.

Two blade props are generally more efficient for our .46 in³ sport class engines.

Using the wrong choice of prop or a lesser powered engine will only exacerbate the negative flight characteristics of the model already described above. To change the flight characteristics, you need to change the design. ie: choose a different model.