First things first its python and qt. To me it feels quite responsive. I also have no idea what "modern" means in this context unless you mean flat with few buttons or options. Calibre can be styled by choosing a qt5 style for your system. If your default is ugly you ought to change it. Such styles can be chosen in your gui configuration menu or via qt5ct at least under linux.

This is what it looks like for me. http://imgur.com/a/px8zF

That said I don't want to start up a whole app just to open a book I usually just open books via a script that wraps calibredb a cli interface to calibre's ebook library. If more than one result is available in response to a query it shows them via rofi. https://davedavenport.github.io/rofi/

It also remembers the last n books that have been opened whether they have been opened in the gui, a script, or otherwise and when called without arguments it returns the list of recent reads which can be selected via rofi. Since I often return to the same several books repeatedly on multiple sessions this is a more pleasant way to access my books.

My script is a bunch of not so awsome shell script. If you want to improve something a better cli experience out of the box for calibre would be nice.