Third party programs slowing down or blocking Vivaldi
Gameguard wich comes with many RPG games...blocks Vivaldi new tabs.
you can open new tab normaly but you can't load any websites on it.
Sandboxie blocks Vivaldi 3.4 (and other Chromium browsers) with Version 86 from starting.
More discussion at https://forum.vivaldi.net/topic/50993/sandboxie-issue/