It depends. What do you think if I say it's 1x slower?
Is it as fast as the original or does it take twice as long?
Is it as fast as the original or does it take twice as long?
People should just use duration instead of speed as you did at the end: "takes twice as long", "takes 1/3 of the time..."
I have played a fair number of incremental games and this quickly became a pet peeve of mine. So many will say things like "2x more" and it will actually be "2x as much". Fortunately, I don't recall any which actually switch between the meanings but it's so commonly a guessing game until I figure it out.