The same thing is happening to us over here as well.
We have 2x 8700 BB's in use and 2x 7100r series devices all running off a BES/Exchange setup. Syncs to the calendar are instant -- messages composed and sent from Outlook instantly appear on the BlackBerry as well. Only read status and deletes aren't syncing in a timely manner. Selecting 'Reconcile Now' on the handheld will update everything normally as it should.
BES version is the latest with all the latest patches.