Freezing when opening messages with "large" attachments

[SANAFABICH]

Hey everybody, and thanks in advance for any help.
So, what's the deal with Thunderbird freezing for like 1 minute every time I try to open a mesage with an attachment larger than 10 MB?
At work I receive photos on a regular basis, and if the sum of photos in an email is, say, 20 MB, after I click on that email so I can read it I have to wait 1 minute for Thunderbird to become responsive again, and then 30 seconds when I try to save those photos into a folder. What gives? I never had this problem before with older versions. I think this has been happening for about 6 months now. 4GB RAM, dual core CPU.

[DanRaisch]

How is the account set up in Thunderbird, as POP or IMAP? Find out using menu path Tools->Account Settings->Server Settings->Server Type at the top right of the dialogue.
How are you connecting to the Internet?
What security software is running on the system?

[SANAFABICH]

How is the account set up in Thunderbird, as POP or IMAP? Find out using menu path Tools->Account Settings->Server Settings->Server Type at the top right of the dialogue.
How are you connecting to the Internet?
What security software is running on the system?

Sorry for the late reply.
POP3
Ethernet Cable on a 20 Mbps connection.
Microsoft Security Essentials.

[DanRaisch]

First, backup your Profile ( http://kb.mozillazine.org/Profile_backup )

Create a new sub-folder under Local Folders. Move all of the messages currently in your Inbox to that new subfolder.

Confirm that you have made the recommended backup of your Profile.

Close Thunderbird and navigate to the \Mail sub-folder of your Profile folder. Delete both the file Inbox.msf and the file Inbox with no extension.

Restart Thunderbird and you should find a new, empty Inbox folder in place. Copy any messages you MUST have in your Inbox from the sub-folder created above to the Inbox. Compact the Inbox after those messages are copied in.

The "Real Fix" entry in this article -- http://kb.mozillazine.org/Compacting_folders#Real_fix also describes this process.