If folks are actually interested in how speed limits are set in American cities, I'd encourage them to read about the process in their municipality rather than absorbing information from message board conspiracy posts. In practice, there are indeed several competing pressures that dictate speed limits.

One source of pressure to keep speeds high is that if municipalities can classify a street as a collector or arterial, they can get federal aid for projects related to the road. As such cities will often want to bring a local road up to collector or arterial status so that they can get higher federal funding for it. (Collectors and arterials usually are larger streets with higher speed limits, though the biggest gray zone is between collectors and local roads and often the biggest difference is indeed the speed limit.) Urban planning discourse about "stroads" in the US is largely a story of local roads that have eventually been classified as collectors or arterials so retain the residential or commercial density of a slower road but are expected to have the speeds and vehicle volumes of faster roads.

(FWIW I first saw this on city-data in the early 2000s and vaguely heard my parents' generation say similar things to this comment, but when I actually started listening and attending local meetings I realize that it's mostly an urban legend that gets repeated to each other.)