I gave it a try a few months ago, but did not work for me. My main issue is that atuin broke my workflow with fzf (If I remember correctly, pressing ctrl + r to lookup my shell history did not work well after installing atuin).

I'm sympathetic, also a longtime fzf user here. I install it reflexively on any system I use for more than a day or two.

This is configurable! I use atuin, but fzf with ctrl-r.

I like atuin but why is it so slow when first opening (hitting up) in the shell?

I'd recommend disabling atuin when hitting up and just leave it on ctrl+r instead

Either it wasn't a design goal or they are stupid. Why don't you tell us?

The right way this would work is via a systemd service and then it should be instant.