|There was an old bug many years ago where stored graphics you'd added to your own collection here slowed posting way down, but I haven't had an issue since it was fixed. No idea if what you describe is related in some way.|
I have about 600 filed messages here, but most don't have any graphics in them. Are the graphics in your filed mail linked to other sites or copied/embedded in your own database here, or some combo of the two?
If they're links and not embedded, perhaps every time folders are accessed it tries to open every link to every graphic contained within.
Purely guessing here but it would explain the behavior.