Of course, my mind glossed over the point that the factor is being applied to the speed, so 300% faster than 20 items per hour is 80 items per hour. That makes sense. It's also analogous to "300% more than 20 is 80".

But then it's hard to make sense of the idea that 300% slower is 5 items per hour (if I'm understanding correctly), since it works differently from "75% less than 20 is 5".

Yeah, "more slower" is indeed a weird way to phrase something. People should just refer to the absolute values, not to the differences. E.g. "1.4 times as fast as …" or "1/2 as heavy as …".

If I read "300% slower", I like to think about time, not speed.

300% more time = 400% original time = 4 times as long = 1/4 the speed