Some thoughts …
I've tried Vivaldi for the past three months on and off. I'm running Windows 7 on a Toshiba laptop. Every time, under a normal install, Vivaldi would constantly freeze or freeze my entire system. Then I tried the standalone installation. (1.0.344.37 (Beta 2) (32-bit)). It works perfectly. (Over two weeks and no crashes). The only thing that I find slow is the actual call to Vivaldi itself. (Forums etc.). Maybe this observation will help the developers in some way.
I can agree with you. I am also a on-and-of user testing Vivaldi when I got a few minutes. In general I am impressed. When slow - I am on Vivaldi forum and blogs. That might just be the serves hosting these services.