On Linux, I prefer to use MuPDF for just opening any epub file. That was originally a PDF reader, but it has been working for some time also with epub files.

On Linux, I prefer to use MuPDF for just opening any epub file. That was originally a PDF reader, but it has been working for some time also with epub files. It renders the pages not only very fast, but also better than any other epub reader that I have tried (all the others seem to make worse choices about the relative sizes of various page elements).