Thunderbird goes non-responsive because your computer is doing something such as your Anti-Virus is scanning incoming mail and has temporarilly taken control.

The timing 5 - 10 minutes may coincide with Thunderbird connecting with server and downloading mail. So, it looks like the Anti-Virus product may be the culprit. Do not disable the AV as then the default one will take over and leave you in the same situation. You need to stop the AV scanning Thunderbird folders on startup and also when scanning incoming/outgoing mail.

Windows updates, Adobe updates, Anti-Virus updates, Game updates etc etc can cause the same type of lag. You may have scheduled tasks occurring in the background which could be causing this issue.

How can I stop the AV scanning Thunderbird incoming and outgoing mail?

What AV are you using? What OS are you using?

Depends upon what product you are using, whilst I know a reasonable amount of info on Thunderbird, I'm not fully conversant in all the various AV products.

You have to open the AV product to see what is available in the various settings as they do vary.

dovg, can you give us an update, and mention your AV software please?