Sync slows or hangs on large bulk messages from Yahoo

Question

I'm attempting to create a local backup of my Yahoo webmail, using IMAP to Thunderbird.
It seems to have worked, except for one Yahoo folder (out of 30), which had the following characteristics:
- 11,500 total messages at start
- about 300 messages that, including attachments, were 2 - 25 Mb in size each.
It appears that the smaller messages have all been synchronized, but those larger than 1.2 Mb have not. Attempting to open one of those larger messages displays the header from the most recent smaller message that was opened, with a blank message text area, i.e. the message cannot be downloaded on demand.
The activity monitor showed, for that (XXX) folder, "Downloading 2 of 300 in XXX". It took 3 weeks to change from '2 of 300' to '3 of 300'.
Other folders have successfully synchronized messages up to 16 Mb in size.
I may have made things worse, or at least more confusing:
After reading that Tb may have performance problems with folders containing a large number of messages, I created, at the Tb end, sub folders for each of the years 2008 - 12 and dragged into them the messages from each of those years. The result is:
- The 5 new subfolders were created on Tb and at Yahoo.
- The 5 new subfolders are empty at both places (and their Properties show 0 messages).
- The 'XXX' folder at Tb no longer contains the messages from 2008 - 12. Its Properties say it now contains 2711 messages. The same folder at Yahoo still contains ail of the original messages, plus a few new ones.
- The Activity Monitor now shows " "Downloading 4 of 80 in XXX". I interpret this as reflecting the departure of the 2008 - 12 messages from that folder, leaving only 2013 - 14 messages to be dealt with. The '4' has not changed in 3 days.
I have also noticed that new messages added to the 'XXX' folder at Yahoo have not been synced down to Tb, even though they do not contain large attachments. I hypothesize that they are queued up behind the clogged sync mechanism.
So something seems to be seriously clogged up, if not completely hung. Various monitors of network traffic show not much is going on.
Tb is set to download everything, regardless of age or size and not to delete anything (as befits a backup).
I am running Tb on an iMac with OS X 10.8.5. I am reluctant to contact Yahoo for support as this may not meet their definition of a mobile device.
I am using the IMAP account settings recommended by Yahoo:
imap.mail.yahoo.com
Port 993
SSL/TLS
Normal password authentication
Maximum number of server sessions to cache: 2 (initially 5, reduced to 1, saw no change, increased to 2)
One suggestion I read for dealing with large attachments was to download the attachment, then delete it from the email. Yahoo webmail appears to provide no method for deleting an attachment while preserving the email. If you forward the email to yourself, the attachment can be omitted, but the forwarded email will carry the current date, losing its original date for sorting purposes.
I would appreciate any enlightenment regarding what is actually going on here, along with any suggestions for getting the synchronization to complete in a proper and timely manner.

Answer 1

actually turn off the global index entirely for the moment. Thunderbird menu > preferences > advanced it is just another thing consuming resources at the moment when they may be needed elsewhere.
Do you see the issue appearing after you guess about a megabyte has been downloaded? see http://thunderbirdtweaks.blogspot.com.au/2010/10/i-cant-sent-mail-networktcpsendbuffer.html