Performance Issue with OWA (large amount of Folder and subfolder)

We encounterperformance problems with OWA.

Indeed, manyusers haveabout two hundredsfoldersand subfolders.

Here's how toreproduce the problem:
Inowa,right click onan email.
Clickon"Move to".
At this point,thewindow listing thetarget folderstakes about40 secondsto display.

Thetests were conductedon the LAN,WAN,on multiple computers(IE7,IE8 andIE9,Firefox),WinXP ProorWin7Pro.
In all casesthe problemappears.

Ifthesame operationis repeatedjust after the first one(within a fewseconds)thewindowis displayedinstantly.

We assumea performance issueonOWAAJAX requests whilelisting folders of amailbox.

How toimprove this?
Can a configuration/parameters changesolve that ?
Has someonealready encountered this problem?
Howhashedone to resolve it?

Thank in advance for your reply.