Slowness of Thunderbird can be with different reasons. Some of the common reasons of slowness are as follows.
1. Due to Add-ons
2. Installed Themes
3. Background Downloads
4. Antivirus Scanning
6. Updating of Software or Windows OS.
You can check all these step by step like., Disabling the Add-ons, Uninstalling the installed themes for Thunderbird, check background downloads if any, check antivirus scanning and for viruses on Windows, check for windows updates.
Hope it works for you.