My Norinco M14 was much harder to operate than either my Garand or Mini-14, to the point that it took two fingers to put it on and the thumb to take it off. I don't use the silly thing, but to me it should absolutely be possible to operate it with the index finger and without breaking a firing grip.
I checked for burrs and greased it, not much better. So I mixed some lapping compound in with grease, applied it the contact points of the lever and snapped it on and off about 300 times, it was still no where close to acceptable. So finally, I disassembled the trigger mechanism and used a stone to remove material from the contact points that were now nice and shiny. The big offender was obviously the point that cams the hammer out of engagement with the sear, it was bringing the hammer back a long way against the mainspring. It is debatable if this action is even necessary, so I wasn't worried about taking too much off. In the end I didn't take too much off, the hammer is still moved by the safety but it can now be operated by one finger without too much discomfort.