Here’s some assumptions I’m making: blade is sharp & balanced & securely mounted correctly, deck adjustment is level, mower drive belt is correct size & routed properly, & tires are properly inflated. Also, your problems do not include the mower actually stalling out, but simply the performance drops off after a few minutes of mowing, and is otherwise ok.
If the engine is making sufficient power, that power is delivered to the drive system. If you then lose power, the loss must be coming from resistance, slop or damage, etc., with some drive system component, it would appear. You can check your blade (mount) & pulleys for damage, resistance, slop, etc., and check the blade brake and disc brake for proper operation. Along with the assumptions stated above, this looks at most everything in that system except for the motion drive belt and the actual trans-axle. Careful not to exclude that motion belt, as this could be a player in your situation.
Then again, your problem starts not when you engage the blades, but only after mowing for several minutes. This sends us back to the engine that seems to be running fine. Simply running the engine in place or riding on the mower without engaging the blades has far less demand on the engine compared to when it’s actually mowing. Is it possible the engine has a low compression issue, & this deficiency is increased once the engine is hot? Yes it could be. If so, could this result in less power delivered to the drive system? Yes again. Could this power loss account for bogging down & losing rpm & an uneven/ineffective cut? Yes sir, it’s possible. And compression can drop due to overheating.
Overheating can occur in a number of ways, such as operating with a restricted fuel flow, a collection of deposits an/or debris covering cylinder cooling fins, valves sluggish/sticking/too tight, low on oil, worn rings, and/or the ignition coil could be faulty, etc.
What it all means is you’re either not making enough power at the power plant, or you’re losing it in the drive system. I would inspect the drive system first and ensure the motion belt is not the culprit. Then I’d check the fuel system for proper venting and good flow and connections, etc., and check the plug to see what it might tell you (if it’s white/dry it indicates a possible fuel restriction issue = may want to service carburetor). For good measure ensure the oil quality and level is good. And then test the coil and check your compression cold, and then each again once the engine is hot.
If, after all this, you have not been able to identify a problem, then it’s likely going to be in the trans-axle.
Hope this might help. Good Luck.