That's normal behavior for the BES. If you read what I posted earlier:

2. After a period of 15-20 minutes of no activity, the BES rescans the mailbox to make sure that nothing was missed or for some reason it did not receive any notification of new items

That means that if you use your BB at all or if you get email in Outlook then the 15-20 minutes starts all over again. 30 minutes is not a huge deal and there is no way to make it faster, except using Reconcile Now.