Experienced this tonight with a group. The slows were acting really wacky. When I got 1 or 2 on me, there was a possibility they'd go away and I'd return to full speed. But if too many stacked, then I just stayed permanently slowed the entire time. This also happened to the entire group. It might have had something to do with slow traps working in conjunction with the shamans casting slow. I'm not entirely sure, but it was undoubtedly buggy. We barely managed to finish the quest at a crawl after about two hours.