What were your workouts like in that period? Identical? Curious if you were doing 4 easy and 3 subthreshold? I think this stuff is interesting to look at.
Once I got onto that, over about 5 months I got faster. The third workout was the key to upping the load. In the end I just got way fitter, having been slower initially after a month or so. And yes I agree, my easy runs are now so easy I don't even really need a shower! But, it really allows me to do 3 solid workouts a week. It's not the workouts themselves that are magic, it's slowly increasing load.
I think everyone is finding in the end, if you can increase that load over a long period of time, there's an excellent chance you will get faster. Those who have come down from higher mileage to this, no suprise it's really not doing much for them. But the reason the majority are improving is if you take the previous 6 months, then their first 6 months of nsm, it finally allowed them to not just stay healthier, but run or generate more stimulation towards their fitness than the previous 6 months.
Same with guys who could already handle more intensity crammed via either workouts or easy runs on 4-5 days, versus whatever they are now doing over 7. They may have been better on 5-6 cramming it in, than 7 days. I've seen a few people like this who running at a harder intensity almost all the days, probably creates as much load as 3x subthreshold and 4x very easy. They just happen to be able to handle that and that is better for them.
But the ramp rate in built is so small, I think it's very hard to guage over such a short amount of time. I lump in people who say they have improved or not improved by the way in that, neither can tell what is truly happening I don't think until quite a lot longer down the line.