Here is an example.
I tried opening a Mobi file that I had converted from PDF. It took aprox. 5 minutes to unfreeze in the reader after I wanted to skip the cover page. At the end it said that a script was a problem and if I should stop the script or something. Once I did this, it skipped the cover page and finally I was able to read the text.
Here is a screenshot (see attachment below) on how high my CPU was during this process. I think there is a bug. I recall in my early days of using calibre, this problem did not happen before. Hopefully it can be fixed.
BTW, my OS is XP Pro with a 4GB RAM -- I have enough memory to convert videos swiftly fast on this PC, so I doubt it's a memory problem on my machine, but I am willing to help troubleshoot it, if you guys have some steps to offer.