Let's say I'm trying to make a web page load faster. Before, it would take 10 seconds to load a page. After some system upgrades, the web page takes 1 second to load.
I'd like to tell the client that
the web page has improved it's render speed by Y percent.
I'm not sure how to solve for Y so that my English expression conveys the appropriate message. Is it Y = 100%(10/1)? So in this case, I've improved the web page render speed by 1000%?
Render time has decreased by $90\%$. The amount of decrease is $10-1 = 9 $ seconds. As a percentage of the original $10$ seconds, this is $\frac{9}{10} \times 100\%=90\%$.