@louis I don't know how you customize your firefox but I certainly feel sluggishness interacting with the vivaldi UI, even when freshly installed. It is true that they have those marvelous tools on the sidebar and such, the customization options and keybindings are great, but the sluggish UI is the biggest reason I switched back to firefox/librewolf after about two days of test drive.
Then I've recreated the vertical tabs/sidebar in librewolf using some userChrome.css trick and Sidebery plugin, and it has served me well since. I find myself not really needing those sweet little integrated sidebar features since my workflow does not heavily rely on a browser, and I've already got the same functionality in emacs.
If you have no UI issues, then I'd say it's indeed a good browser, although proprietary, it gets the job done pleasantly.