One way to significantly speed up many (some?) functions in Calibre is to put the temp folder on RAM, if you have plenty of memory in your computer.
In Linux (I use Ubuntu) this is easy by using tempfs. To do this I added this line to the file /etc/fstab
tmpfs /tmp tmpfs nodev,nosuid 0 0
This will use up to half of the available RAM and if needed also the swap partitions for the folder /tmp. As before, other unused memory will be used as disc cache. To have /tmp on tempfs may also good if you have a SSD, since it will reduce the number of writes to the SSD, increasing both performance and lifetime of the SSD.
Google /tmp fstab tmpfs
for more info. Add the name of your Linux flavor.
This may also significantly speed up other programs that use /tmp a lot.
I believe something similar can be done in Windows? Then you can still take advantage of your 64 bit OS and extra memory, while running 32 bit Calibre.