I love Vivaldi, however, you can't deny that it is noticeably slower than for example Chrome.
Regardless, I'm still using Vivaldi everyday since the pros far outweigh the cons. This is coming from user-experience on a brand new MacBook and several Windows machines with 16GB RAM + 4.4GHz and 5.0GHz clock speed. It's a performance issue, and is not a result of your components.
The machines and Mac were used on a 500/50mbps line, so no, it has to do with a Vivaldi's optimization.
Chrome feels smoother and more responsive