[Dan__]

I found some ePubs on the internet that a loaded into iBooks. I experienced the following problems when trying to read them:
1 - loading and page turning is extremely slow, verging on unresponsive
2 - cannot bookmark a page
3 - current reading location is not saved when closing the book

I suspect that there is something wrong with the internal format of these books but I have no idea what it might be.

Does anyone have an idea as to possible causes, and is there anything that I could do with Calibre, perhaps by way of a conversion, to end up with an ePub that is well-formed?

Thanks in advance,
Dan Essin