Instead using 4 seconds down to 2 seconds, think about 4 seconds down to 1 second or down to 3 seconds. Plug those numbers in instead of 2 and see what the percentages look like.

If you are thinking of an improvement over 4 seconds, then 2 seconds of new response time means that 2 seconds was trimmed off of the original time. If you have a 4 pound roast and you cut 2 pounds off of it, you cut off 50%.

But if you are thinking of something like throughput, then the rate has increased by 100%. That is, instead of doing one ‘Login’ in 4 seconds, now you can do two. The throughput rate has doubled; or it has increased by 100%.

It’s not so much a matter of your formula as it’s a matter of what you’re measuring.

Tom