A lot depends on the server hardware and how many users are on your Domino servers. Have you met or exceeded the minimum requirments for server hardware? Are there more than 500 BB users? How many Domino mail users?
Without knowing any of that, one thing that may help is to change the frequency of polling that the BES server does against the Domino mail files. I believe the default is 30 seconds. If you increase that to 2 minutes, you should see a decreased load.