Here are the specs: http://www.tractordata.com/lawn-tractors/001/2/0/1202-cub-cadet-1529-engine.html
I got this lawn mower used in 2012 (it was in very good condition). It has somewhere around 500+ hours now. I've done all the maintenance on it. It has new spark plugs, air filters are changed often enough, and oil changes I've been doing once or twice per summer.
The gasoline I've used has varied throughout the years but I typically use 89 unleaded with 10% ethanol and it typically doesn't sit with gasoline in the engine because I have a fuel cutoff switch I installed. I've also installed a new fuel filter. I have ran higher octane fuel (91?) in it which I suspect wasn't good for it.
Starting last year, after an oil change and a few other maintenance tasks (possibly prior), I noticed it was not running as well under load as it used to. In taller grass (even short grass sometimes) it bogs down. It can barely make it up a little hill with the blades spinning. I have to go up a little at a time. It never used to have this problem. The engine starts great, never misfires, etc. Overall it's smooth sailing except for under load.
Thing's I've tried: With a neighbor's help, I've recently checked the valve spacing so those should be good. We actually replaced one of the valve arms because it was bent (not sure if that was caused by us checking it or if it was actually bent prior). The engine still runs the same after fixing that.
I've sharpened mower blades (I have to do it fairly often or else mower won't run good enough).
I've replaced spindle bearings. They don't turn easily but there's not a ton of resistance either.