Receiving, deleting, and sending messages is extremely slow for 1 of 2 users on HP desktop computer.

Thunderbird had been working great for both of us until after my wife stopped checking her emails for a couple of months. When she later opened the program her inbox started downloading hundreds of emails (very slowly) until her inbox contained well over a thousand. I started to delete the unwanted ones but it went very very slowly. I finally whittled it down to a few hundred but the program continues to run very slowly in all phases. In one instance, it took 11 minutes to delete 39 messages. Downloading and sending are not much different. I've done virus scans, checked settings, restarted the computer etc., all to no avail. And when we switch users, my Thunderbird email runs perfectly. I also contacted my internet provider (Spectrum) but they said it had to be a program issue. How can I restore my wife's email to normal function? Thanks, Joe S.