The toxicity and performance of an employee are distinct.

Riffing on this Hacker News comment.

It's a 2x2:

Low Toxicity / High Performer: someone everyone can agree you should keep around.

High Toxicity / Low Performer: someone everyone can agree you should fire as quickly as possible.

High Toxicity / High Performer: some people will think that it's worth it to keep them around, despite their toxicity, which means you'll keep them even when they are actually a net negative.

It's easier to see the direct positive effects than the indirect negative effects.

Low Toxicity / Low Performer: someone that might make sense to keep around; for example perhaps they are more willing to do thankless or grungy, gap-filling tasks that no one else wants to do.