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.