Well hopefully someone with experience with that specific model or who at least had a WSM for it will weigh in. I know she large tractors have closed center hydraulic systems where the circuits are independent of each other. If yours is closed center ignore the rest of this. TractorData lists it as open center. Most of us here deal with open center hydraulic systems: one loop that daisy chains around the everything hydraulic on the machine. The easiest to solve is when the system has gone into relief for some reason, which causes low flow on an open center machine, but that causes slow response on everything, not just one thing, so that doesn’t sound like your problrm
The typical chain for open center is loader first, rear remotes second, three point last. “It will lift anything” sounds like you have pressure from the pump and it’s not getting lost down the line so that would seem to eliminate a lot of things such as leaking fittings, lines, seals, etc. “It’s just slow” sounds like a flow problem.
So back to the open center order of plumbing. If the loader works normally in both lift capacity and speed that would seem to eliminate anything to do with the pump, screen, filter as you know the input from the pump is adequate in both pressure and flow. So is the flow from the loader’s power beyond port restricted? If it’s going to the rear remotes and they work normally, it’s not restricted. So then you’re on to the power beyond port from the rear remotes to the three point. If it has external cylinders on the three point all that may be outside the rear housing and easy to trace.
If all the plumbing is external you might be able to trace the order of plumbing visually. If you can’t do that, you’ll probably need a WSM to verify how it’s plumbed, then start at the pump and follow the daisy chain around to the point the restriction is causing slow response.
Also noted you said you adjusted the flow control under the seat. For most of us with smaller machines that flow control under the seat just affects the return rate on the three point flow so it only impacts the fall rate and has no impact on the raise rate. Yours might be different.