Drop a JPG and get a WEBP back. WEBP usually lands around a third smaller than a JPEG that looks the same, which is why it has become the default for photographs on the web. The conversion happens in this page, so a photo you have not published yet does not have to visit anybody’s server on its way to being smaller.
This page loads no libraries at all — the work is done by the browser itself. Your file is read on your own machine and never sent anywhere — open your browser's network tab and watch while you use it.
Usually between a quarter and a half, for a photograph at comparable visible quality. Flat graphics and screenshots vary more widely. The result line gives you both sizes so you can see what actually happened rather than trusting an average.
Every current one, and has done for several years. The exceptions are old versions that will struggle with the rest of a modern site anyway. If you need to support them, keep the JPG and serve WEBP alongside it with a picture element.
Yes — this is a lossy conversion from an already lossy file, so it is a second generation. For publishing on the web that is normal and invisible. For an archive copy, keep the original: converting back to JPG later would be a third generation and would start to show.
No. The picture is redrawn from its pixels, so EXIF including the GPS position is dropped. For images headed to a website that is usually what you want; if you need the capture date, read it off the original first.
Everything here is free and works the same way — in your browser.