I've found Vivaldi is becoming slower and slower with each update to the point that I'm starting to lose confidence in the browser itself.

https://forum.vivaldi.net/search?term=slow&in=titlesposts&so...

http://www.pcworld.com/article/3052560/browsers/vivaldi-brow...

https://thequo.wordpress.com/2016/04/23/one-glaring-problem-...

etc...