Having lived on both one-way and two-way streets over the years, I have to agree - with caveats.
It all depends on what the one-way and the surroundings look like. If you’ve a narrow one-way with lots of parking and barely enough space to pass if someone double-parks, and trees, and buildings crowding in - yeah: traffic will slow down.
OTOH, if you’ve multiple one-way lanes, huge setbacks, not a lot of pedestrians, etc. - it’s gonna suck. And for some reason this typology sucks a lot more in Toronto, than say…New York.