The article/slideshow I linked is not a specific scientific study that was done in London, it’s a summary/aggregate of other studies that are referenced at the end of the slideshow. It was a study summary made for London, but the science behind it is a lot more general.

I’m from Belgium and from my own personal experience, I find that well done low speed zones really do improve the flow of traffic. Cities in the Netherlands have been at it for probably over 2 decades, Antwerp has followed their example since about a decade and now other cities in Flanders are copying Antwerp’s homework. When done well, it works really well and almost noone wants to go back to how it used to be. You’re right in that coordinated traffic lights are a big part of why the traffic flows much better, but in congested streets, a lower speed is needed to keep that flow going.

In Belgium we also have a big example of how to not do street renewal/traffic improvement programs: Brussels.

The evidence of studies says that you are wrong.

Here are some key points from a study summary that was made for London: 20mph zones do not appear to worsen air quality and they dramatically reduce road danger. They also support a shift to walking and cycling, generate less traffic noise and reduce community severance. In 20mph zones vehicles move more smoothly with fewer accelerations and decelerations. This driving style produces fewer particulate emissions.

https://content.tfl.gov.uk/speed-emissions-and-health.pdf