Welcome. It's impossible to tell at a distance what the issue is, but we can speculate.
First thing I'd do is put the C-dale on a work stand and verify wheels are spinning freely with no resistance, tires not rubbing on the chainstays, properly centered in dropouts, etc.
If they're OK, then on to check wheel hub and bottom bracket bearing condition. On a 1986, chances are you need a rebuild asap, as any grease from that long ago is no longer functional.
Also, hubs that are overtight will cause binding and require more effort to pedal. It could be something as simple as that.
Friction shifting + freewheel (probably not a cassette from 1986) shouldn't make a difference as far as effort. Properly tuned systems should be equally easy to turn.