We have a 2014 279D with about 2,500 hours on the machine. When running in continuous flow to operate a brush cutter, the lift arms pickup very slowly. It seems like it gets slower as the hydraulic temps increase to above 160F. I can't guarantee we have the right fluid in as we haven't done a change since purchasing earlier this year. With little experience running a machine in continuous flow, so I don't know if this is to be expected. I would assume it would slow the lift arms, but not to this extent. Probably moving upward at like 1" per second when fully pulled back.
If you don't know what's in it then I would strongly suggest to change both the hydraulic oil and the fitler. Use the recommended SAE 10W oil and although you don't have to use Cat oil at the same time don't put cheap stuff in it......... HeavyEquipmentForums Report back with your results.
When completing the change, do you worry about what's left in the drive motors and cylinders contaminating the fresh oil?
You will probably have to change the oil a couple of times in short order to make sure everything is flushed, unless you want to go to the trouble of draining the cylinders, drive motors, & lines the first time you drain it.