i have an lx shifter connected to an slx 2 ring specific, top pull, standard type derailleur. shifting from the small to big ring is incredibly hard and takes a thumb of steel. cable is clean and lubed. all the resistance seems to be in the derailleur spring which is mighty powerful. is there something i'm doing wrong?

If you disconnect the cable, can you move the mech by hand? You should be able to hold the cage and pull it outwards with reasonable ease.

Is the cable routing the right way? Are the adjuster screws on the mech been set properly? Is there any debris in the mechanism in the mech that may be preventing it from moving smoothly (small stone perhaps?).

Are the cables new? Inners and outers? Are the cable lengths correct?

Is your shifter internals cleaned and lubed? Spray some GT85 into the shifter to see if that helps loosen things off…

What ring size different is there? Could the stiffness be down to a very large jump between the rings?

I'll assume it's not worn.
What size rings are you changing between.
Have you got the mechs height right.
Is the angle correct.

The front mech has a massive job to do. Manufacturers spec quality rear mechs on bikes but the front has a far bigger job to do and is almost ignored. For example if you ever look at Shimanos electronic system you'll see that it says front shifts will shorten the battery life.

Edit; Samuris tip is a good one, can you move it into place by hand with the cables unattached?

is the cable virtually slack when you're in the small ring?

It should be the stop that's holding the mech in place not the cable and shifter.

I've found if I have too much tension in the cable when in the small ring then changing to a bigger one gets really difficult.