High CPU reading epubs and PDFs in Linux

10 February 2026

Recently I had a peculiar problem with some ebooks. Both Calibre and Okular were obscenely slow when I tried to open these documents or switch between pages. Converting the epub to PDF didn't seem to make any difference. The programs would freeze up for tens of seconds at a time while one CPU core ran hot. I poked around in the epub's contents and couldn't see anything out of place. It wasn't like it was hundreds of MB of scanned pages or anything like that.

I found the solution by accident: install a proper font for Chinese characters. Although these texts were English they had Hanzi sprinkled liberally throughout. In some parts of the UI they were rendered extremely poorly and in other parts they were missing entirely. I'm not exactly sure why this hits the CPU so hard. I guess it was doing some sort of search to figure out what to display instead? So I ran:

sudo zypper in google-noto-sans-cjk-fonts

...and now I not only have all the characters, but everything is nice and fast. Aren't computers grand?

Tech Tidbits Blog by Thomas Karpiniec
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