VM Blast performance Issues -- Server performance nearly dies during 10 minute VM blast to 150 mailboxes

I have FreePBX setup for a mid-size company (about 70 extensions with another 80 voice mail boxes)

The man with his name on the door loves to send 10 minutes voicemail blasts to all 150 employees.

As soon as he finishes, the load average on the server spikes to over 20. Some calls get dropped. During normal operations everything is fine

Any advice on how to slow down the process of sending 150 emails and making 150 copies of the recording?