I had a similar problem.
It did not go away until I did this:
1 Close Vivaldi
2 find your vivaldi folder on your home/user page and delete it.
It then forgets it was ever started and you can start a new "first time"
Yes the UI is slow on my old laptop(AMD Turion 64 X2 Mobile@2000MHz; ATI Radeon X1250) and that is the biggest issue so far.
Still, i like where all this is heading and maybe i'll finally have a real alternative to the old opera(which i'm still using)…
ps. The panel toggle could've been thinner(2px max) 🙂
I just commented on a topic about translation of vivaldi.net : https://vivaldi.net/forum/suggestion-box/236-the-official-translation-localization-thread?start=20 but again, if there's a need for a Polish translator, I'm eager to help.