While it is likely your prop, one does have to wonder what kind of person would have setup the prop/engine/toon combo to only produce 5k RPM. Since the only answer to that would appear to be someone who doesn't know what the hell they are doing, I would first look at some minor maintenance things to rule them out before going prop shopping.
1. Is the prop damaged at all? Even minor blemishes can reduce its efficiency and make it harder to spin.
2. Slap on a new fuel filter. If nobody has changed the fuel filter in a fairly long time, it's possible the engine is not receiving as much as it needs. Yep, I've experienced this one myself.
3. Spark plugs. Both my Yamaha and my Mercury 4S engines lose horsepower with older plugs. I have them on an every other year schedule for spark plugs, but the Merc in particular got kind of doggy when I went into year 3 with the same plugs. I suppose I put about 50-80 hrs a year on the Merc back when that happened, but it was noticeable. I am thinking it was about 300, maybe almost 400 rpm difference. It seemed to run fine, smooth and all, but was simply lacking a little in its normal get up and go. And visual inspection of the plugs was deceiving. They looked used, obviously, but they weren't fouled or anything else that would have you wanting to replace them. Heck, I finally changed the spark plug on my weedwhacker a couple weeks ago and was downright amazed at how much more power it had...and again, the plug wasn't obviously fouled.
Come to think of it, when I first bought my Lund with the Yamaha, the first time I dunked it on the river it struggled to even get on plane. Like seriously, it was almost embarrassing at how underpowered it was. It ended up needing new spark plugs, and I remember correctly...the engine only had around 120 hours on it. Otherwise, it ran smoothly...just didn't have any power.