Also, you might try backing up a bit. Using the 1.8.3 firmware, see what the experience is like with a half dozen books.
The Original Kobo works well with native Kobo books and reasonably well with epubs. Bear in mind, this is a very simple ereader -- no bells and whistles, just a plain reading experience (and the ability to borrow library books with ADE DRM). Once you become familiar with what the Original Kobo is good at, then you can try adding more books to it.
I presume you are generally trying to open epub and not pdf files?