This went from 131 to 76. The 70% calculation was (start-end)/end. Is this the standard method, because it seems like you would divide by the start timing.
Similar with compression. If a 8kb file becomes 2kb, the compression factor should be 0.25x.
When we say 1.7x faster, what is a fast? How does fastness increase? It's nearly 2 fasts instead of one?
I think the compression sample you gave is much better. Multiply the original file size by the factor to get the new, compressed file size. This is much better than a statistic that I hear a lot about the compression in a columnstore database we use, where everyone says "7-10x compression", because that was published first and is popular.
I'd prefer to see something like your compression example. It took 8 seconds before, and now takes 2. My optimization makes the operation take .25x the time.
The one that bothers me is when people say something got x percent slower.
Sort of like losing the amount of time it takes. If I went from $100 to $50. I would say I lost 50% of my money.
Part of me feels like a 50% performance increase is another way to say that I lost 50% of the time it once took.
This is all subjective on my part.