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