This came up only a few days ago in the thread
https://www.mobileread.com/forums/sho...d.php?t=248762 (relevant posts start at 10 or so). We are still looking at the best way to ameliorate things.
Bottom line: the problem is a combination of the size of your library and the virtual libraries column. For the moment, don't use the virtual_libraries() function and you should be fine.
EDIT: You really should be running the 64bit version of calibre, given you have a 64bit OS.