I have an extremely old Windows PC (10 years old, I'd guess) and Calibre runs beautifully. I love Calibre. - I really do.
I also have a Toshiba NB205 with 2 megs Ram, and Calibre has always ran slow on it. I use Ubuntu on it, and keep both Ubuntu and Calibre current. This slowness has nothing to do with offloading, or reloading.
I have a 160 GB disk in the Toshiba.
I confess that I am no Ubuntu master, and have just sucked this up for 2-3 years, and will continue to do so. But it is really just about all I use this netbook for.
I also have an Astronomy program on it that runs like a little demon. So I don't think it is hardware. But I'm open to any advice.
Any suggestions must keep in mind that this is all I've really used Ubuntu for was this netbook, which has two jobs: Calibre and Stellarium.
So I'm no whiz kid. Feel free to talk slowly to me.
My Library is 10G, but it is a copy of the same library on the Windows machine.