Your email client gets slow when loading MBOX files with duplicates. I can suggest one solution you should try to remove duplicates using
SameTools MBOX Duplicate Remover. It cleans large duplicate MBOX files quickly, reduces their size, and makes them load faster in any email client without slowing your system. It keeps your original data and finds duplicate MBOX files and removes them in one attempt. This tool supports all Windows OS versions, including 11, 10, 8.1, 8, and 7, without requiring any additional applications.
Visit here:
http://www.sametools.com/duplicate/mbox/