[DNSB]

This thread seems to explain why I haven't been seeing the slow page turns that many are reporting. I use Sigil to reformat most of the sideloaded epubs I download. Part of my standard setup is splitting on chapter boundaries. Looking at the last set I modified (Thorne Smith from gutenberg.au, the chapter size shown by 7Zip vary from 11KB to 32KB uncompressed. Compressed size is about 35-40% of uncompressed size.

If I get the file in a non-epub format, I use Calibre to convert to epub and then clean up the file with Sigil. Takes a bit longer but makes my reading experience much better. Just one item with Sigil -- Save! Often!! The search and replace code tends to crash and dump all your work since the last save.