Vivaldi is clunky and slow because the entire user interface (tab bar, address bar, title bar if enabled, etc) is a webview rather than native-ish controls like all other browsers.
I used it for six months and it had some things I really liked about it, but there were some really frustrating bugs that would pop up each release. I eventually got tired of it and had to move to another browser.
It's a shame, because I like the attitude and spirit behind Vivaldi.
