I have a Nikon F system circa 1967 and the following Nikkor lenses: 35mm f/2.0, 50mm f/1.4, 135mm f/3.5. All are in near mint to excellent condition, and all are part of the same original kit.
The odd thing is that the 135 lens focuses extremely stiffly, while the 35 is like butter...almost a little too smooth (not really...it is quite luxurious). The 50 feels "normal" to me. In between the other two.
Given that these lenses were probably used extremely rarely (the 135 is dead mint without a single scratch), I am wondering if Nikon designed their teles to focus more stiffly than their shorter lenses. The 135 3.5 is a small lens, so the weight of the glass should not make a difference. I have Canon 200 and 300 that focus more smoothly...but no other experience with Nikon glass.
I highly doubt it, but I was curious. Given that the 135 3.5 is a $50 lens *tops* in any condition, I won't be having it fixed...but I was going to look for another one. However, no point in that if it will be just the same.
It's probably the type of lubrication they used, and lack of use may make things worse.
I just had to have a 1961 Rolleiflex serviced I knew the original owner well - he'd only put a few, he said 6, films through it, he also had a Yashica which he used in preference, he wanted to "save" the Rollei.
Lack or little use can be a bad thing in some ways, also lubricants and the materials used to build lesnses change.
I think it's inconsistent quality control when it comes to manufacturing. I've had several AI Nikkors since bought new from 1979 to 1984.
One of them has had a loose element since I bought it. Two of them (50mm and 28mm) focus very smoothly. A third (105mm) focuses slightly stiffly. A fourth (20mm) is very stiff.
Optically, they're all fine, but it's obvious that final checks on each lens were not performed.
In the case of your lens, it's hard to say. The age of the lens might or might not play a role. I'm just speaking from my experience.
The reason for the different focusing feel is probably the state of the lubricant. Over time the lubricant either dries up and becomes very stiff or it starts separating and and becomes very gooey and ultimately leads to oil on the aperture blades. Remember the lubricants of the 60's & 70's were not up to the quality of today. You could try too dissassemble the lens yourself and clean out the old grease and put a small amount of high temperature wheel bearing grease in its place.
Yes, I think it's the lubricant usually. I had Focal Point work on my Summarit many years ago and I asked them to try to alleviate the "stiction" (sticking and friction) that I was having a problem with. They tried to address the issue by changing lubricants and were partly successful, though it's still not as easy as I'd like.
I also have my 100mm f/3.5 RF lens worked on -- it's the black/chrome model that does not have click stops on the aperture. Because it was old, the aperture moved too easily. The repair facility told me they had to try different lubricants until they hit on the one that kept the aperture ring where it should be, but also didn't make it too hard to move.
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I have a similar vintage and condition Nikkor 135/3.5 and it's stiff also. My guess is that your lens probably was never used much at all, so the lubricants have stiffened up. Had the lens been used on a regular basis (or maybe even just exercised a bit) you might not have had the problem at all. How the lens was stored is a factor also.
Only thing to do is to clean the old lubricant out and relube it.
Lubrication and wear both contribute to smoothness. Old lube sill stiffen things up a bit and if a lens takes a bump on the focusing ring it can knock the ring just slightly out-of-round which will also stiffen up the focus.
About the only thing you can do for the bumped lens is remove the old lube & lap the helical & unless you want to spend a LOT of time(tedious & boring) with it, it will always be stiffer than it was before.
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Are some lenses designed to focus more stiffly? 
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