A slowbanned user has delays forcibly introduced into every page they visit. From their perspective, your site has just gotten terribly, horribly slow. And stays that way. They can hardly disrupt the community when they're struggling to get web pages to load. There's also science behind this one, because per research from Google and Amazon, every page load delay directly reduces participation. Get slow enough, for long enough, and a slowbanned user is likely to seek out greener and speedier pastures elsewhere on the internet.