Speed demon

Near-synonym: Speedy Gonzales

Being a speed demon in video games is OK, but out here on the roadways in the real world, you might injure or kill your crash victim and end up in prison.

The reason they're speed demons at checking off work tasks is that they do the work carelessly and don't bother to check their work for mistakes before submitting it. Maybe they'll get fired eventually, or the company will be plagued with recalls and returns and end up going under.