I'm very sure hellbanning doesn't affect comments prior to the ban--do you know something I don't?--but when new comments by that user are posted they are instantly marked dead. The slowdown is part of the hellban, the whole point is to make the site unpleasant so that the user leaves instead of coming back to harass. With the user believing the site is slow as tar and no one responding to them, it's quite effective. The downside is that it loses its effectiveness as it becomes public knowledge.
It's a problem from the point of view of a writer wishing to ensure that his writings will be read.
From the point of view of a reader, however, having information about what will likely get moderated be scattered rather than gathered in one place decreases a reader's exposure to text written by people who have not read a lot of comment sections.
