whenever thor fights people a lot weaker than him, to leave them a chance, they easily dodge and land many hits without thor being able to react. So like, daredevil or wolverine look a lot faster than thor in a fight and quicksilver runs circles around him
Other than that, relatively speaking, characters like superman (or others from DC like wonder woman) generally show more speed more consistently (like a lot more than quicksilver types)
So when thor fights superman or "fast" characters on Vs threads, he gets pummeled even if his other stats can compete. Hence "slowdinson"