4 Yes, 5% is proprietary code, but only related to the UI and full auditable, in the same community they even show how the user can modify it for his likes and needs (at own risk). Vivaldi hides nothing.

Performance currently v.5 is good, better as the other browser I have Otter and Palemoon, even faster than Edge. Security is relative in every browser, Chrome and Edge are not private, but secure, Firefox and Vivaldi are a lot more privat and secure, all they protect against fishing, fraudulent webs and all of them use a sandbox system currently.

I've tested nearly all browsers and Vivaldi is the best and most advanced of all I've used, at least for the moment. I don't know what Google do in the future in the store, but novaday most extensions in the store in Vivaldi are redundant and not really needed. But yes, it's almost a full online suite and not for someone who needs only a browser to consult the mail or to post in a social network.

It is made also for this, including a mail client and feed reader, but it also have a notepad with Markdownm which works also as multiclipboardfrom the context menu, inbuild translater not from Google, multilevel tab-stacking, splitscreen, ideal for study and video conferences, clock and pomodore timer, screenshot, web-panel to ad websites in mobile view, History panel with calendar, graphics and stadistics, mouse gesture, fast-keys and key chains, Calendendar, QR code, ........etc. Apart an UI fully customizable, even with a panel for insert CSS files, you can put the icons, tabs, adress - and .other bars everywher you want, or hide them.

There is no other who can do this.