It's never that easy to calculate.

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.