I haven’t looked into this specific case, but most of the time the limiting factor is other traffic. You’re not traveling at full speed the whole time. If a lower speed adds 10 minutes to the average trip, but it reduces 9 minutes’ worth of traffic, you’ve only lost net one minute. A lower speed limit will often reduce traffic because the speed-up-slow-down behavior is reduced.
Personally, I have driven around the Netherlands a fair bit and this sort of thing does seem to be roughly true for the median case. It can definitely be annoying when the streets are empty, though. For those journeys you’re obviously losing a fair bit of time.
I have a long stretch of road near me that used to be 50 km/h and is now 30 km/h.
If you take that road during rush hour, yes - there's no meaningful difference in time spent. You'd be going traffic light to traffic light slightly faster.
The problem is that I personally use that road (in a car) more to pick up/drive my wife from and to early and late shifts at work than during rush hour, and this makes it take a significantly longer time. Like, I am not complaining at all, but it takes something like 20 minutes instead of 15, so like a 33% increase. And then again on the way back. But in the end being lucky with the traffic lights is still the main point.
Personally, I have driven around the Netherlands a fair bit and this sort of thing does seem to be roughly true for the median case. It can definitely be annoying when the streets are empty, though. For those journeys you’re obviously losing a fair bit of time.