Slow performance with large folders over the Internet

[Shawn Heisey]

Dovecot package version is 1:1.2.15-7+deb6u1.  It is in Debian 6.0.10,
using the Debian package.

The server is in my basement at home, and is exposed to the Internet so
I can fully access my mail from anywhere.  I use IMAP for reading mail.

I have a number of folders in my mailbox that have thousands of messages
in them, from mailing lists.

When I'm at home, I have a LAN connection to the server.  It goes
through a Cisco firewall that limits the connection speed to 100Mb/s.
In this situation, I can open a folder with 25000 messages in it, click
on the next unread message that Thunderbird did not know about before,
and within a second or two, the message will download, allowing me to
view it and reply.

When I'm at work, with highly variable network latency between
Thunderbird and the server, doing exactly the same thing takes a LOT
longer.  I have seen it take as long as 15 minutes for a single message.
 If I open a folder with only a few messages in it, it is fast.

The server is not overloaded -- I can log into it with ssh and use "mutt
-f" to open a folder directly.  Loading thousands of messages into mutt
takes a while, but I have no difficulty using the ssh connection and
running commandline programs.

This suggests that the IMAP communication between the server and the
client involves a large amount of back and forth communication when the
message count in the folder is high, possibly something for every
message in the folder.  It happens quickly on a LAN but crawls on a
connection with high latency.  I can understand it taking a few seconds
longer on a high-latency link, but it takes minutes.

I do plan on building a new server and migrating to Dovecot 2.x, but I
haven't had the time to work on that.

Is this a known problem? If so, is it fixed in 2.x?

Thanks,
Shawn