Clutch

Two possibilities that I can think of. 1) is that the clutch itself is old worn out. It would simply be slipping. Or 2) it could possibly be that oil is leaking from the rear main seal, getting on the clutch, and causing it to slip. Have you seen any oil spots under the truck?

Test the clutch. Put the truck in first gear, let it out then floor the throttle. Do the same in second gear. Does it still have to catch up?

If yes the clutch is simply worn out. Pedal pressure doesn't mean much because that is just the pedal pushing on the pressure plate springs.