First, as far as I remember, there is a major factual mistake (possibly a mistranslation) in the article you quote. What happened is that speeds were reduced by 20 km/h in non-urban areas. I do not recall, and cannot find historical records of, speed reductions to 20 km/h, or in urban areas during the 2014 winter pollution peaks. Specifically, the rule was:
The maximum permitted speed for motor vehicles is reduced by 20 km/h:
- On motorways: [a decree] limits the speed to 110 km/h on motorways instead of 130 km/h.
- […] limits the speed to 70 km/h on road sections limited to 90 km/h and to 90 km/h on sections limited to 110 km/h.
Source: Oise prefecture, 13 March 2014 announcement. (Oise is a district (département) just outside the Paris region.) My translation.
Maximum speeds are legally reduced to 110 km/h on motorway segments where the normal maximum is 130 km/h, to 70 km/h on [roads] where the normal maximum is 90 km/h, and to 60 km/h on the [Paris] ring road [where the normal maximum is 80 km/h].
Source: 12 March 2014 announcement from the Paris police administation, 4th body paragraph, my translation. The 110-to-90 reduction applied in the Île-de-France region too.
The influence of speed on air pollution depends on the type of vehicle and the type of pollutant, but the studies I've found consistently find that for cars, the fuel consumption curve has a U shape: low speeds and high speeds result in more pollution per distance traveled than medium speeds. However, pollution doesn't always directly follow consumption because the efficiency of after-treatment can also vary.
A 2020 study by Emisia relayed by the European Environment Agency evaluated fuel consumption and emissions for speeds above 90 km/h. It found that at these speeds, a higher speed means more fuel consumption and more CO2 per distance traveled, as well as more particulate matter for diesel engines (apparently the study didn't record PM for gasoline engines). For NOx, emissions increase with speeds for diesel engines, but for gasoline there is an optimum around 115 km/h. Gasoline cars emit more CO at higher speeds, but diesel cars emit less (due to a catalyst which is more efficient at higher speeds due to the higher temperature).
The environmental effects of changing speed limits: A quantile regression approach by Germà Bel et al., Transportation Research Part D, vol. 36 p. 76–85, 2015 studied actual data for NOx and PM pollution before and after policy changes in Barcelona. I haven't read the full article (which is paywalled). Here's a passage from the abstract:
In 2008, the maximum speed limit was reduced to 80 km/h and, in 2009, a variable speed system was introduced on some metropolitan motorways. (…) We find that the variable speed system improves air quality with regard to the two pollutants considered here. (…) However, reducing the maximum speed limit from 120/100 km/h to 80 km/h has no effect – or even a slightly increasing effect – on the two pollutants, depending on the pollution scenario.
A 2021 report by Cerema (a French public agency) compared fuel consumption and NOx, PM and CO2 emissions depending on the average speed, ranging from 10 km/h to 130 km/h. They used averages over different types of vehicles based on the typical fleet composition in 2020 and anticipated fleet composition in the future. They found that for an average car, fuel consumption, CO2 NOx and PM emissions all have a U shape and are lowest per km traveled around 70 km/h. On the other hand, for a truck, slower is always better, even down to 10 km/h.