I've been doing a Japanese broadcast run through and it just seems like it's easier.
The clear rate in Sasuke 1-6 was 42%, 30% if you remove the Sasuke 6 outlier.
The clear rate doesn't dip below 50% again until shin Sasuke.
I don't think you can group 7-17 together because of the difference in difficulty over that period. 7's Stage 2 was 90 seconds, and had an unfailable back half. 17's was 65 seconds yet had the Balance Tank and Metal Spin.
Similar story with 5 and 6. The Tackle Machine wasted a few more seconds than the Narrow, which with that stringent a time limit, made a huge difference, especially when you consider the fastest clear time on 6's Stage 2 was like 5 seconds.
I would definitely say 5's Stage 2 was the most stringent time wise, because it seemed like there was zero room for error or hesitation, but 15-17's was a lot more difficult in raw terms because the obstacles themselves were a lot more failable/fatally punished mistakes more. Which is more difficult depends on the competitor; bigger and slower competitors would defo find the former harder, but for those where speed isn't an issue (e.g. Nagano who can beast Spider Walks), the latter has more room to be caught out unexpectedly.
7-9 was for sure easier than the original Stage 2, because realistically all they had in addition was the Chain Reaction, which, while it did cause a couple of fails during that period, didn't warrant a 40-50 second time bonus. You could tell from the way that competitors were rushing the original Stage 2 yet on Sasuke 7, they were really taking their time, because they could. Especially considering that obstacles like the Spider Walk, Goren Hammer, and Wall Lifting became much more of an issue when competitors were forced to go fast; people got stuck under walls, lost balance on the beam, slipped on the spider walls, etc.
On the topic of clear rates, you can't really compare them because the original era's Stage 1 was so much easier that we had a far worse overall Stage 2 field who were doomed to fail there no matter what. Whereas the Golden Era often had clears in the high single/low double digits from far more elite competitors who had a higher chance of clearing Stage 2.
Generally when there are easier Stage 1s, Stage 2's clear rate is lower because more fodder competitors get there. Look at Sasuke 34. We had 24 clears on Stage 1 and only 9 on Stage 2, then with 35 the clear rate was 5/8 because most of those Stage 2 fodder competitors got eliminated by the tougher Stage 1, and only the stronger competitors, who inherently had a better chance at making Stage 3, cleared.