It is possible that your hydro could be getting weak. A poor pump would cause reduced lifting capacity, and slower lifting under load.
That would also cause the tractor to slow down when it hits resistance.
Very true Jim. I once owned a CC that had a worn hydro and as soon as I got to even a slight uphill, it would slow by maybe half to 2/3 of it's speed. The rotors & faceplates were so worn it was leaking lots of pressure under load. This as an MTD Cadet so not worth expense or time to repair. I found another unit. On the older hydro's, you can machine both parts & renew them.