I'm actually using Slimjet on both Windows 7 and Linux. It's ability to turn off WebRTC to avoid IP leak when you use a VPN. It is also up-to-date (I think) with the latest stable Chromium build which is also used by Chrome. Has a few other goodies packed in it's settings also. Before I only used Firefox (well actually using Cyberfox since it's faster with it's custom AMD build) which you could just type in the address bar "about:config" without quotations then find the media.peerconnection.enable and make it false (which you can search for) instead of eyeing it out while in about:config.
On Slimjet and Cyberfox, you can just easily uncheck it in the settings. Slimjet is the only Chromium built browser that I know of that supports turning off WebRTC (I don't know if Opera can or not since I don't use it anymore). Vivaldi, if not now, will most likely support turning off WebRTC. As for Slimjet being the safest?... It's probably safer than any other Chrome like browser. Since Opera is using the Blink engine, I guess it could technically fit in there. As for the fastest browser? It really depends on your system config. However I am under the impression that Cyberfox is the fastest Gecko engine (Mozilla), and Slimjet is the fastest Blink engine browser (Google - a few others helped though according to wikipedia).