If you only use TB to read text-only newsgroups, I can't see any reason for significant bandwidth fluctuations. But if you were downloading large binary attachments, it's possible there's one that is failing to download, but is repeatedly attempted, in the background, each time you access the newsgroup.

There are plenty of apps to monitor network traffic, some built into the operating system, that you could use while performing actions in TB or any other program.

Thank you for your reply, 'sfhowes'

There shouldn't be any binary attachments, but I'll keep a close eye out for same from now on.

I've started to use Bandwidth+ to help me monitor the situation. https://apps.apple.com/app/bandwidth/id490461369

D.

Have you marked any newsgroups for offline use? Right click the newsgroup name, select properties and the synchronization tab. This would result in every post to the group being downloaded. That could add to a substantial amount of data on some USENET Newsgroups.

Account settings > Synchronization and storage will show all subscribed groups and their corresponding offline use setting if you are subscribed to more than a few.