example/jsperf/ui.js: report both `slower by xx%` and `slower by a factor of X`

[GerHobbelt]

(shown as 51% slower (✕ 2.0)) as the computer is better at calculating both relative indicators than we are, and we can use our brain for the conclusions rather than the calculus.

(Of course one might want to see 'faster by a factor of X' instead, but this was more useful for my discussions.)

Don't know if you want this or want to keep the jsperf example as-is.

Anyway, thanks for all the work and the quick updates of the repo,

Cheers, Ger