I never had performance issues, but I don’t read huge books or anything. There seem to be some online converters, but I agree that is not the most ergonomic. Maybe calibre-web could also do the job, with you connecting to it through your phone’s browser.
Never heard about the kepub
format but I’ll look into it, sounds cool.
I have a Surface Go 1 with 8 GB RAM running Aurora-DX, which includes the linux-surface kernel. It works great, and I find that modern KDE works quite well with touch, even though I mostly use it with the type cover attached. I only use the surface connect port for charging, but I do use the single usb-c port with a usb-c hib, and it works well. The Fedora atomic distros work great on little machines like that.
Edit: I’d add that Bluefin is the same with Gnome.