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

Tire and brake particulate is a real air quality impact that is either worse with EVs (for tire particulate because they are heavier) or mitigated, but still present for EVs (for brake particulate because regenerative braking reduces but does not eliminate the use of physical brake pads). For ICE cars (which are still the dominant form of car when looking at what's on the road today), efficiency might go up in terms of fuel consumed per mile at a steady speed of 90 kph, but urban driving involves a lot of stopping and going, and reducing the speed limit reduces the amount and intensity of acceleration being done, which is where emissions are going to be worst. Also, the existence of 30 kph zones encourages alternatives to driving, whether that is low-no emission modes like walking, biking, or transit, or moving car trips away from the 30 kph zones to other areas (there's a potential negative externality to the latter, but if done well, this can move car airborne emissions away from where a lot of people live and play and towards corridors and areas that are designed with uses that are more tolerant to poor outdoor air quality (e.g.: industrial parks, generally lower density areas, etc.)). That's not to mention road vehicle noise pollution (sometimes considered part of air pollution, but most times not) and safety benefits that come alongside the air quality benefits discussed in this article from having fewer and slower cars

in general, efficiency goes up till ~90km/h.

Are you talking about fuel efficiency or total emission though? The efficiency might go up but the emission might still go up, it might be less emission per km/h but maybe not per km.

In Berlin, only 3% of all registered cars are pure electric cars (numbers from 2024).

Speeds of 30 km/h lower the emissions of nitrogen oxide from cars with combustion engines. This happens because of significant reduction of pollutant-laden acceleration processes.

The reduction of emissions from such a speed limit is based on more uniform traffic flows.

Lower speeds reduce road danger

Are extra casualties and reduced freedom for people who aren't driving worth the few minutes saved for a minority of people?

Slow speeds make public transit a little more attractive to people on a time budget.