If your firmware version is 5.16.3 or higher, you will need the hf (hardware floating point) version of KOReader. As of today, the nightly version is
koreader-kindlehf-v2024.11-206-gb8a31ef18_2025-02-15.zip. The stable site does not include any kindlehf versions. I originally used the version from Marek's khf site
https://fw.notmarek.com/khf/koreader/