Every time I try use Vivaldi I encounter how incredibly slow the UI is. Are all Vivaldi users running it on specced out desktops? Or is it just ao lineux UI latency issue?

Same, even on the absolute highest end machines. It has been a few versions since I've given it a go and I otherwise like it but the perf decline over stock Chromium, sometimes randomly appearing, is what has always steered me away again in the past.

I'd guess it's the price for custom UI and customization features they're adding