[Kajetan]

Originally Posted by KevinH

FWIW, Qt has a number of "blurry font" related bug reports similar to this one:

https://bugreports.qt.io/browse/QTBUG-76872

Perhaps one of those is the culprit?

No, the culprit was a setting of my NVidia GPU driver

Fast Approximate Antialiasing (FXAA) has to be turned [Off] for Calibre.

Since then ... every letter is displayed sharp and easy to read.