[parkher]

As to the speed, the reading / writing speed of the card remains the same. Internal or portable. Sure, ext4 perhaps is more efficient and faster than fat32. But encryption is used for internal, so that slows things down a bit.
In e-reader, the main purpose is reading. Writing speed is not critical. Indexing of dictionaries (or libraries) is not critical either.
You only need to have fast enough reading. And those cards are fast enough for HD video playback. Certainly fast enough for a page of text. Besides, caching is used in smarter reading software. You can install any reader app you like, you are not limited to the default one. Although the default one is pretty good, especially for pdfs.