Yes people saying “50% more” is very common, and usually easily understood to mean 1.5x.

I think something like “0.5x slower” is just conventionally not used, even if it’s understandable. It’s a difference in common usage between percentages and x-factors. One reason might be that x-factor implies multiplication, not addition; that’s what the “x” stands for. X-factors are typically used to say something like the ‘the new run time was 1.7x the old one’. Whether it was slower or faster is implied by whether the x-factor is below or above 1.0. Because x-factors are commonly used as multipliers, and not commonly used to say “1.5x more than” (which actually means 2.5x), it’s pretty easy for people to misunderstand when someone says “0.5x slower” because it looks like an x-factor.

Now, the same argument could apply to percentages. A percentage is also a factor. But in actual usage, “50% more” is common and “0.5x more” is not; and “100x faster” is common (usually to mean 100x not 101x) while “10000% faster” is not common at all. So language is inconsistent. ;)

All that said, using an x-factor as a pure factor, and not a multiply-add, is less confusing and more clear. Saying “The new runtime is 1.5 times the old one” leaves no room for error, where “The new runtime is 90% slower than the old one” is actually pretty easy to miscalculate, easy to mistake, and easy to misinterpret. The percentage-add is also asymmetric: 90% slower means 0.1x, while 90% faster means 1.9x. Stating a metric as percentage-add makes sense for small percentages, and makes more sense for add than subtract once the numbers are double-digit and larger.