There isn't much inside that can cause your problem, but there is a bunch of stuff on the outside.
Note that I'm not discounting the possibility that the problem is inside, but the odds favour the pedal to transmission linkages as the source. Only after the linkages have been proven to operate freely and correctly do you consider opening a hydro.
Maybe disconnect the linkage right at the hydro? Might tell you where the drag is. However, if there’s a bad bushing in the linkage, the resistance from actuating the hydro might be just enough resistance to magnify a bind in the linkage (bushing).
Is it just as hard to move the pedal with and without engine running?