[hughdbrown]

.9 * x = 0.5

x ln 0.9 = ln 0.5

x = ln 0.5 / ln 0.9

x = 6.5788

So decreasing runtime by 10% 6.5788 times results in the code running in half the original time.

[Dylan16807]

I think their number is the right one. Ten percent faster is not a ten percent decrease in runtime, it's about a nine percent decrease.