Calibre has a built-in 'Polish books' feature which will remove unused CSS. I ran your book through it and it did an impressive job:
Quote:
### Removing unused CSS rules
Removed 23754 unused CSS style rules
Removed 3813 unused CSS selectors
No unused stylesheets found
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It also reduced the filesize by about 400k. HOWEVER, it didn't make opening it in KOReader any faster. I wonder what else might be causing the extreme lag?
There is an easy fix for opening it quickly, though: Convert the book from epub to epub in Calibre. It will then open instantly.
Or alternatively, if you ever don't have access to Calibre, you can long-press on the file in KOReader and select Open with... MuPDF. You won't be able to change fonts, use style tweaks, etc., but it will open without delay.