SMTP escalation

There are four SMTP nodes, mapped to a virtual server. When bulk mails (up to 1 million) are routed through the F5/virtual server, the latency is high and takes forever to process. On a direct connection to one of the SMTP nodes, the throughput is much better. This has been going on for over a year and is most noticed at month end when the bank needs to send bulk mails to her customers.

Question:

1. What could be causing this problem
2. How many SMTP request can the F5 BIG-IP process per second
3. How do i resolve this please ?

Grateful for your time and response.