Email is very slow not only in deleting but in retrieving new emails off of the server (TMS).
When deleting an email or group of emails Thunderbird takes up to 45 seconds and will frequently display "Thunderbird Not Responding".
Wšykne wótegrona (8)
That sounds like a network slowness problem. Thunderbird sends a request and must wait for a response from server.
Correct me if I'm wrong but once emails are downloaded to Thunderbird everything that is done (e.g. copy, save attachments, delete emails) is done locally, subject to the speed of the local device. On my computer the machine's speed, according to Norton, is not the problem. On my phone I'm using Samsung email app and I have no such problems.
If POP, that is basically true. If IMAP, then deleting requires online access. Lots of it.
Try to start Windows 10 in safe mode with networking enabled. Does the problem go away?
Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.
Running safe mode still took about 40 sec to move 14 emails to the delete folder. Deleting individual emails takes about 10 to 12 sec.
Is your PC running an IMAP account? If so, that could explain what you're experiencing.
POP3 Account
See if this helps. https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems