I've noticed the same thing. When you open a book in the Library, it seems to do something with the file before it passes it on to the display application. I don't know what it does, but it's the thing that pops up that "File is too large" dialog box, and it does take longer the bigger the file is. Maybe it's taking the md5sum of the file and using that to index into a database, or something.
No idea about the mobi files; I didn't even know they could be read on Pocketbook devices.
Last edited by rkomar; 07-02-2012 at 02:46 PM.