Front derailleur has too much resistance I got a mountain bike on the cheap because of this repair. I'm not experienced with bike repair but hoped I could figure it out.

The front derailleur, a Shimano STX, isn't shifting and it appears to be because there's too much resistance to it swinging outward. When I try by hand to pull the derailleur where the cable lands, it's too strong to move. Only when I get better leverage, such as pinching the cage in my cupped hand, will it move. 

Everything is all oiled up.
I opened up all the adjustment stops so it's not them getting in the way.
The cage is parallel with the chain so it wouldn't seem it's at the wrong angle, which would create resistance by joints rubbing the wrong way.
I tried weaving the cable through the clamping screw a different way, hoping it could provide more leverage, but that didn't help.
When I give the cable more slack, the shifter is able to pull it until the slack is gone, so it doesn't seem to be a cable problem.

It seemed to me that only the spring remained. It's resistance is too high, maybe? But how would a spring have more resistance than it was designed to do? And if that's what I need to try, I don't even see how I would get it off if I wanted.

Thanks for any help.
The FD needs to have a firm spring, because it has to operate against the full chain load when pedalling.  By comparison, the RD operates on a piece of chain under no tension, and the RD is putting the tension on that length.

So you need to check the inner cable that runs from the handlebar shifter down to the FD mech.  If its bright-shiny silvery then that's good, but if its dull and rusty then that's not good.  A new inner cable is cheap, a little fiddly to fit, and will "bed in" over the first month of use.

Second is the outers - could be someone's fitted a new inner cable but not changed the outer(s) and they're scraggy on the inside.  You should be able to feel this when pulling the inner back and forth with your hands.

Finally - the handlebar shifter.  Its not unknown for them to clagg up with rust.  They're reasonably expensive to replace.  More-so if yours is integrated into your brake lever.  Needs to be the same number of speeds as the rear cassette.