Mine takes a little bit to open some books that I have converted too. I have not timed it, but I would guestimate it at around 30 seconds to a minute depending on the book. I have mine set to the default. I have not really played with the settings in that. I converted about 20 books when I first started and haven't needed anymore since. I may try to play around with the settings when I do my next batch and see if anything improves.
The best thing would be to find some books that work really well and see what the settings are for those books. Then try to match that.