I recently replaced the valve lifter on the Panther. It was one of those relatively small pointed-end jobs as used on smaller bikes, and it took a lot of effort to lift the valve lifer on a running 600, plus the movement was minimal too.
I replaced it one of those much beefier ball-ended jobs (original British item 2nd hand off Ebay, and not a monkey-metal modern repro) and it works a treat.
Simple answer, bigger lever = less effort.