A 40% reduction in speed only causes a 5% increase in travel time? Are the majority of car trips spent sitting at stop lights??
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.