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I've had this before and never been able to work out why . basically my mates shifter for the front mech is very hard work for the thumb. It's on a 20yr old kona and apparently was nice and light in the past. SO..
the mech itself moves fine although spring tension feels pretty hard when operated with the hand it's not due to a lack of lubrication or stiffness in the pivots . The spring has not been moved or adjusted and surely would get weaker if anything over time ?
The cable is clamped and routed correctly and the cable inner and outer all smooth running. Finally the shifter works smoothly when not connected to the mech so ...What causes this ? I've had it once before and never found a solution apart from changing the mech . I assume it's the very stiff spring but could they become less flexible with age?
Dirty/worn, wrongly routed cables and or stiff mech pivots.
Much drag with cable routed but disconnected if not it has to be the mech?
Also check frames cable stops, the inner cables can sometimes rub on these were they exit.
I had this on my old road bike and although the cable felt fine changing the inner sorted it instantly
Often the outer cable can get a groove in it which is not evident until the cable is under tension. Change the outer.
I had this with my inbred.
I'd clamped the cable the wrong side of the nut.. Difficult to really explain but there's a very small grove at the clamp where the cable should run. The difference was amazing!
Olds mechs seize and get really stiff quite often.