Hunsie
I am using the latest avast version and Thunderbird 1.5.0.8.
I am experiencing no slowdown in receiving mail or deleting mail.
I am fairly certain that your problem is with Thunderbird. Thunderbird does not handle file space very well and it requires that its users take care of it.
When you delete a mail message in any avast folder it is not deleted at all, it is simply flagged to be not displayed any more, but it is still there and the folder just gets bigger and bigger. It is also copied to the trash folder (which gets bigger and bigger). So it is important to empty the trash folder too.
The only way to really delete mail messages in Thunderbird and to get control of the space it is using is to regularly compact the folders. (If you check the Thunderbird support forums you will find the folks there giving this advice to many of the problem reports).
It is also not a good idea to keep all you mail messages in the Inbox, if you have a lot there consider moving them to another folder.
There is an option in Thunderbird (Tools > Options > Advanced > Offline & Diskspace tab) to have Thunderbird compact folders. I use an extension called Xpunge which provides an automated combined trash emptying and compacting function.
Bottom line - I think you will see a considerable improvement if you empty the trash folders and then compact all your folders. If you care to search this forum using the keywords Thunderbird and compact you will find quite a few others have.
