By "ebook file" I meant whatever the kind of file that Calibre loads up.

So you have narrowed it down to a problem loading your file, not a problem loading Calibre itself, right? Calibre loads quickly when it's not loading your file?

You have also narrowed it down to a slowness bug in Calibre, not a slowness bug in your general operating system, right? Since Calibre loads quckly, it's only loading your file that's the problem, and other programs don't have this issue?

There's something Calibre is doing that is being slow, and it's doing it when it loads up your file.

That thread you linked talked about BONJOUR a lot. Bonjour is an APPLE program that would have self-installed at a time that you installed some other related Apple product such as iTunes. I have, in the past, found it to be buggy, and I have had slowness problems with Bonjour. It is a networking program which inserts itself in your network stack, and it has sometimes had problems on certain kinds of networks which slows down my computer when it makes certain kinds of network calls. For instance, I had a work PC which, when it was joined to the work domain, would take 60+ seconds to shut down because it was trying to synch my profile to the domain at shutdown time, and Bonjour would get in its way because Bonjour was trying to shutdown at the same time and it was blocking the network stack. My solution was to uninstall Bonjour. Bingo, quick shutdowns again.

Run APPWIZ.CPL, see if BONJOUR is installed, and remove it.

If that solves your problem, then you have indeed had the same problem as the people in the thread that you linked. The problem is that somehow Calibre is doing something on the network and it's trying to either talk TO bonjour or trying to talk VIA bonjour and the buggy-ass not-properly-QA'd bonjour program is getting in the way and causing the slowness.

Please note that Bonjour likes to reinstall itself each time you upgrade an apple product. Lately I've been letting it live on my system because I keep upgrading iTunes (I need iTunes to make backups of my iPhone), but any time I have something that looks like a network slowness problem, the first thing I do is uninstall Bonjour and see if that fixes it, because that's been my experience in the past.