I'm the author.
I have never questioned the clarity of that sentence, at least until today :-)
By "0.5x" I mean "0.5 times or 0.5 multiplied by the reference time" where "reference time" is the inference time of OnnxRuntime. So I'm actually meaning "50%".
I think the expression "0.5x slower", taken by itself, could be misleading, but in the context of the original sentence it becomes clearer ("while being only 0.5-2x slower", the "only" is important here!!!).
But I think the general context of that sentence defines its meaning. I am referring to the fact that under no circumstances could my project, with its premises, be in any scenario even a single millisecond faster than OnnxRuntime. Then in the second paragraph of the README the goal of the project is stated, which is precisely to trade off inference time for RAM usage! Obviously combined with the fact that the performance data is clearly reported and that I repeat several times that the generation of a single image takes hours or even dozens of hours.
However, given the possible misunderstanding, I will correct the sentence in the next few days.