My first thought was wear between the tang's pivot end and the spring. I have an 8OT (Schrade USA) in 1095, bought as-new some years ago. I've noticed if I don't keep it pretty well-lubricated, it'll shed a fair amount of dark swarf from the bearing surfaces between the tang and the spring. If you oil the pivot and the oil seeps out black in color, that's a sign of that metal swarf in there. In general terms, I've noticed this with carbon steel folders overall (blades and/or springs). Knives in stainless steel are more wear-resistant in the pivots, and I don't see it as much in those.
So, the sluggishness could be due to the accumulation of that dark metal 'dust' and/or just dirt/grime/gummy oil in the pivot. OR, if there's more wear, it might be due to the shrinking of the radiused end of the tang, due to much heavier, long-term wear. Either way, I'd make sure to clean the knife thoroughly (dish soap & water), exercising the pivot to work any swarf or grit out of it. Then dry it all out and make sure it gets adequately oiled.
The old Schrades had that 'Swinden Key' pivot construction as well. So, it might also be possible that could be a problem, if there's excessive wear in there. I think they were prone to wearing out faster than full-through, pinned pivots. Not much can be done to fix that, aside from completely disassembling the knife and installing full-thru pivot pins, as done with most other folders.
