One last test to see if it's the link. Repeat test 1 with the gun upside down. Does the smoothness of the slide operation go away? If turning it upside down does terminates the smooth function, the link has stretched and is not pulling barrel down quite early enough in the cycle timing. That would vindicate HiBC's observation.
The reason I think this drag happens when the case is in place and the gun is upright is the extractor tension against the brass is likely enough to prevent gravity from assisting the barrel to drop out of locked position. It's hard to imagine how the extractor tension alone could add 10 lbs to your effort, though you can always polish the inside surface of the extractor behind the claw and round its corners a little. Same for the inside back surface of the claw, but just don't dull the tip of the claw.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Patron Member