Hmmm - there's a similar issue with single book conversion, the dialogue box is slow to open, see ==>>
Conversion Performance issues.. Maybe they have a common cause.
Could you try a single book conversion to see if it is similarly slow.

try running calibre with no plugins, see ==>>
calibre command
Needless to say its not happening here. Windows 64bit version 2004, calibre 64bit version 5.2.
BR