What looks like the relevant table has a summary line saying "geometric mean: 1.45x" so I think that in this case "45% slower" means "times are 1.45x as long".

(I think I would generally use "x% slower" to mean "slower by a factor of 1+x/100", and "x% faster" to mean "faster by a factor of 1+x/100", so "x% slower" and "x% faster" are not inverses, you can perfectly well be 300% faster or 300% slower, etc. I less confidently think that this is how most people use such language.)

What would 300% faster mean?

If the original process took 30 minutes to process 10 items, how long would the 300% faster method take?

300% faster = 400% original speed = 4 times as fast = 1/4 the time

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

And that's where the confusion comes from: according to this, 300% faster refers to 300% more speed, whereas 300% slower refers to 300% more time.

It’s a fair point, that way of expressing it is always a bit confusing. Is it the original time plus 45%? Is it 45% of the original speed?

I think it is easier to understand in terms of throughput.

So 45% less work per unit of time, so 55% of the work.

0% slower means "the same speed." The same amount of seconds.

10% slower means "takes 10% longer." 10% more seconds.

So 45% slower than 2 seconds is 1.45 * 2 = 2.9 seconds.

I guess it is clearer if expressed like "Native application took only x% of WASM equivalent".