I switched over from Pentadactyl to Qutebrowser last year and it's way snappier. And with the experimental WebEngine backend, I haven't run into any rendering or performance issues at all. It also has a vertical tab mode, although it isn't possible to have sub-tabs like Tree Style Tabs.
I'm going with luakit or qutebrowser when I feel I can't continue using firefox 50 that I have on update freeze. Those projects are so promising and can't say I dislike the idea of having a hackable browser on my fingertips. I'm planning to keep Vivaldi as my secondary browser for multimedia and other similar needs.
I switched over from Pentadactyl to Qutebrowser last year and it's way snappier. And with the experimental WebEngine backend, I haven't run into any rendering or performance issues at all. It also has a vertical tab mode, although it isn't possible to have sub-tabs like Tree Style Tabs.