[okular] [Bug 359932] Okular is very slow at opening some EPUB's

[tidux]

https://bugs.kde.org/show_bug.cgi?id=359932

tidux <jon at dorsal.tk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|REPORTED                    |CONFIRMED
                 CC|                            |jon at dorsal.tk

--- Comment #21 from tidux <jon at dorsal.tk> ---
I am running Fedora Kinoite Rawhide amd64 (immutable-base distribution)
currently on Plasma 5.22.  I have installed Okular 21.04.2 from Flatpak.  My
system is a Ryzen 2500U with 16GB RAM and /home on btrfs on a SATA 6Gbps SSD. 
Under default performance settings, Okular uses 100% of a single CPU core for
approximately five seconds while opening the file, and then another four
seconds per individual page turn.  This is the single worst performance I have
ever seen of any epub renderer on any system.  I attempted to trade memory
usage for performance by disabling "Enable transparency effects" and setting
"Memory usage" to "Greedy" in the Performance tab of the "Configure Okular"
menu.  The file took over ten seconds to open, a process which was still
pegging a single CPU core.  Page load time was decreased by 25% to three
seconds, and pages that had already been rendered once opened near instantly. 
This suggests a pair of bugs:

1. The parser takes far too long to load an epub file or a portion of it into
memory.
2. The renderer is extremely slow, and should probably be replaced with
QtWebKit or QtWebEngine.  The entire point of basing EPUB on HTML is that
performant generation/parsing/rendering libraries are universal on every system
capable of making a modern TLS connection to obtain such files.  We should use
one. 

To reproduce, simply open the following public domain EPUB file in Okular:

https://gutenberg.org/ebooks/10

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