"painfully slow" is entirely relative.
A 25hp should push a boat
like that (the 13-15 range) around 15-20mph.
On a 17 mako, it should push upwards of 20-25mph....
A lot depends on the prop and trim. But you're looking at around 20mph no matter what.
Ps: the fact that its a tiller motor
, I would be looking at the smaller skiffs. Look for a V type of hull. Flat hulls are nice in a river, but you will outgrow it very quickly.