What right do you have to come into Diamond Hollow, guns a-blazin'... Destroying the automated turret defense systems... Killing thousands upon thousands of slimes... Stealing all their currency... A vandal. A thief. A murderer. Let's see how many more charges we can rack up.
Don't think of the NPCs as "cheaters" or "not cheaters". I can easily beat them most of the time if I swing fast and have good equipment or powerups. Think of them as 'pacers'. There'll always be one of them messing up all the time, and one of them that's pretty good on each hole.
To get the best hole time; Save up money (using the 'magnet' and double-coin powerups if possible). Buy the best driver, iron, putter, shoes, hat, and ball. Don't waste money on the 1000 or 2500 items. Buy the powerups; PowerSwing, SuperRun, and SuperStop, every hole you can afford it, and don't use SuperStop unless you have to.