[svenlind]

Thank you for your replies. It seems that the main difference, for me, is that the portable is a 32 bits program.

I tried a few conversions with "normal" but still quite large books measured by filesize, and the differences in conversion times were not very clear to me.

I then found the largest book in pages, The Complete Works of Rudyard Kipling,
which according to the page count plugin has 4500 pages. It was converted from epub to azw3 using both the normal Calibre and the Portable Version:

Normal Calibre: 5 min 30 sec.
Portable Calibre: 6 min 8 sec.

I think I will migrate to the portable version, but still keep the 64 bit version if I should encounter a very complicated book.

I have 16 GB ram in my system but the use of memory was almost identical in the two tests.