Since you’re asking for faster, your going to need to subtract. The difference then gets divided by the “old” number for reference to the “before” quantity.

You decreased the time it takes by 99.61%, you increased the speed by 25636%. (Taran’s original calculation was a little off). The final result took 0.39% of the original time, and was 25736% of the speed of the first program.

Maybe if you take a less drastic change of times (not speeds) from something like 11 seconds to 4 seconds, the numbers won’t be as scary.

Your “percentage change” involves a difference expressed as a percentage of the original value.

And your statement of reducing the time is how I’d say it. Dealing with negative time seems a bit odd, but if you wanted to express it in terms of money and went from $11 to $4 it might aid the understanding of the idea.