I gave zellij a try recently. It has a lot going for it... but ultimately it's not nearly as 'tried and true' and programmable as tmux. There's no significant speed difference and some of the defaults (like the "Zellij" label on screen) are rather annoying and surprising when you'd expect a multiplexer app like this would be aimed at the minimalist crowd. A positive outcome of 'rethinking X' project by building a new version is finding new features that can often be implmented with scripting or plugins in original X. I found a few things in zellij that I liked and found ways to do the same in tmux. I rebound some keys to make resizing and pane hopping much much easier than what zellij could offer.