Why would Thunderbird regularly hang for 10 seconds

I've been using the Thunderbird e-mail client for years on three different operating systems: XP, Vista, Windows 7.

On all of these operating systems, it has always had the problem of e.g. I'll be writing an e-mail and suddenly it will just freeze, I can't type, and 10 or 20 seconds later it will "come back" and I can type again. When I look at the task manager when it does this, the CPU spikes to 100% during these times and the guilty process is Thunderbird.

The problem is not always the same:

• on XP it started out fine and these black outs got longer and more frequent
• on my Vista installation it would happen only once every 20 mails
• now on a new installation of Windows 7, it happens almost every e-mail I type and often when I pull the scrollbar down it will freeze, "(Not Resonding)" will come up in the title bar, and I have to wait 20 seconds before I can use it again.

What could be causing this? Has anyone else had this problem with Thunderbird?

From mozillaZine Application not responding :

Symptoms

Thunderbird stops responding and may use large amounts of processing power.

Solutions

In some cases, the problem can be caused by a mail summary file (*.msf) file being corrupted. This can occur when your folder is mildly corrupted due to not compacting it regularly. A quick and dirty fix is to:

1. Exit Thunderbird.
2. Find your profile folder.
3. Open the folder containing your stored messages. For example, if you are using the Global Inbox, this would be the "Local Folders" folder inside the "Mail" folder. Otherwise, it's a folder named after the mail server, inside the "Mail" folder. For example, "pop.gmail.com" if you're using gmail.
4. Select the *.msf files and delete them.
5. Restart Thunderbird.
6. Wait for Thunderbird to recreate the mail summary file(s).

If that doesn't work, it may be because your folder is too badly corrupted. See the second part of the "Compacting doesn't seem to work" section in Compacting folders.

If the problem occurs when sending a message, try re-configuring your anti-virus program so it does not check outgoing messages for viruses.

In some cases, the problem is caused when Thunderbird has trouble detecting and filtering junk (spam). This may be caused by a corrupted training.dat file. It may be necessary to delete this (Tools -> Options -> Privacy -> Junk -> Reset Training Data -> OK) and retrain the [Mail Controls].

Perhaps it's some kind of add-on? Try writing a couple of emails without any on.

If that solves your problem, try turning them on one by one to see if you can recreate the issue.

Thunderbird is lovely but resource-consuming.
On my station it hangs exactly as described in your example when it receives new messages and the antivirus (currently AVG) scans them.
IMHO Thunderbird hangs while waiting the return of the message by antivirus.