Luvstolol,
Hydraulics slowing when warm indicates a flow loss in your system.
Some possible areas for this to occur are:
Bad seals on the cylinders. Usually if the seals are worn enough to notice change in speed you would have also noticed the bucket drifting or dumping while loaded.
Leaking relief valve. This will cause excessive heat and causes the problem you are describing.
failing pump. as the oil heats up the pump slips (leaks internally) which causes the problem you are describing.
Some tests you can run:
Cylinders: Fully lower the FEL and hold the valve like you are trying to lower even more. Do either of the cap ends of cylinders get hot?
I know that accessing components on skid steer is challenging but can you remove enough panels to access the pump & relief valve. Same thing here. dead head a function and see if one of these components heats up faster that the surrounding components. flow loss under pressure creates heat.
good luck.