Thunderbird stupid about X traffic?

[Michael ODonnell]

I use Thunderbird at work and it's OK - its
performance never struck me as being notably
good or bad.  I connected to the work LAN from
home today using OpenVPN and asked Thunderbird
(as an X client of my home machine) to do
its thing, but it's taking about 10 minutes
to completely paint the window and then every
update is just horrifically slow after that -
it's completely unusable.

At first I thought it was my ComCast connection
or some problem with OpenVPN but when I use
Mozilla in the same situation it's tolerable.
I tried a direct SSH tunnel (w/o OpenVPN) but
no differences.  Anybody else seen Thunderbird
behave poorly this way?