580 slow on a hill

Went to move my '86 case 580 SE up a slope/hill earlier this day. It felt like I had to rev it quite a bit higher than usual to get up there but was still a lot slower. Went in 1st gear. Outside temp is about 10c/40f. Could this be due to some oil not being warmed up? I used the machine for about 3min before moving it.
Thanks for your input!

580 slow on a hill