Firs impression is that You are running wrong size propeller, it has typical pitch /dia figures
for a 90 hp engine. But after checking the evinrude quick reference guide looks like the diameter is right, smallest available is 13. Strange though for such small HP.
Try to borrow a few other make/model of props and further play with the trim, height and weight distribution, that should do the job, it should go faster. Good luck!
fun on a boat is inversely proportional to size...sort of anyway