If you run a website, WebP is the easiest speed win available. The same picture at the same visible quality, at a fraction of the size.
WebP typically produces files 25 to 35 per cent smaller than JPG at the same visible quality, and it supports transparency like PNG does. Every current browser displays it.
The catch is older upload forms and some desktop software that still refuse it. For your own website it is safe. For submitting to someone else's system, stay with JPG.
Page speed is a ranking signal, and images are usually the heaviest thing on a page, so this is one of the few optimisations with an immediately visible effect.
Every current one does. Very old browsers do not.
Yes, with the same tool, though converting twice costs a little quality.
No.