[quietbritishjim]

> if you are mistaken, I'd have to observe that this is a spectacularly unclear way of communicating an increase.

I absolutely agree with that, and in practice if I ever see that turn of phrase with anything more than 100% then I assume that they are using it the way that you're thinking of. But I maintain this is not the literal meaning. At the same time, I'm not saying people should be subtracting 100% to make the number mathematically correct, that would definitely be bizarre. Instead, I'm saying they shouldn't be using such a weird turn of phrase in the first place, so the headline should simply be "Deep learning on CPU 3.5x as fast as on GPU"

> I've worked in various places Cambridge and London for the past 18 years and, as I say, have never encountered it.

You have really never encountered an item in a supermarket saying "now 20% bigger!"? Thinking about it now, they're usually charging the same amount as the old size (otherwise it's not much to brag about really) in which case they use the vastly clearer phrase "20% extra free", but I'm sure I've seen the former phrase.