I wonder how hard it would be to make our own kepubs.
https://wiki.mobileread.com/wiki/Kepub
According to that they are just zipped and are similar to epubs. I guess the trick would be to find a DRM free kepub and check the differences in the file format. And hope they don't have a mandatory digital signature.
Any one know of a DRMless kepub around?
Then again maybe it's possible to just rename a epub to kepub... That wouldn't explain the speed difference (unless the slowness was deliberate attempt to discourage sideloading, or if the kepub was being rendered based on its extension using a different engine).