I think that there are still issues with the browser being a bit slow. Compared to chrome, Vivaldi is somewhat slower loading webpages but not loading google.
A new version was released yesterday however Vivaldi is still very much in a technical preview stage which many could say is basically a developer version and not an alpha or beta version. Many of the improvements will come on the front end and behind the scenes as each update is released and bug fixes, functions added and fine tuning are done. I can still remember when Chrome was first released it was pretty much the same way.