If you open a .mobi in Calibre it takes tens of seconds, on a reasonably fast system, to open small (<2meg) books. I'd agree with the other comments here - Calibre is great in terms of it being powerful (web server to serve up books directly to a kindle without having to plug it in, search is ok once you know how to use it) but has a terrible UI (I shouldn't have to google to discover how to search my local library).
I've been using Calibre-Web[1] as a frontend for the Calibre database, visiting it with my Kobo Auro H2O.
It will do neat things like convert to Kindle & email to the Kindle address with a button press as well so I can give access to my mum and so she can seamlessly read the epub books I have in my library on her Kindle.
It doesn't look great on the Kobo, but I can navigate pretty quickly to download a book, and I hardly ever have to touch calibre itself.
It's pretty easy to get set up running headlessly on a server somwhere.
I liked FBReader until I realized it wasn't displaying the blank lines used by the book series I was reading to separate sections in a chapter. Undoubtably the ebooks' CSS was bad, but other readers, like my Sony e-ink device, displayed them OK.
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