I'd suggest generating a debug log with this batch script:
https://www.mobileread.com/forums/sh...63&postcount=2
run script > wait for calibre to load > close calibre > see log generated in the same folder as the .bat file.
this thread got me curious so i just did that... and after looking in the log I found that an emblem formula was calling a deleted custom column, causing some start delay.
deleted the rule > now Calibre loads faster
edit: now that I remembered the "restart in debug mode" option in preferences menu, this seems easier