I came across this useful article in the BlackBerry Technical Solution Center:

Overview
Latency is the time between the initiation of the transfer in a sending process such as when an email message is sent from a BlackBerry smartphone, and the arrival of the first byte in the receiving process.

The following factors are possible reasons for latency:

• The time it takes the BlackBerry Enterprise Server to poll for, discover, and queue a new email message.
• The bandwidth available between the BlackBerry Enterprise Server and the wireless network.
• The current traffic volume in the wireless network.
• The BlackBerry smartphone network coverage.
• The mail servers’ responsiveness when accessed by the BlackBerry Enterprise Server, both in server response time and network connectivity between the two servers.

This is probably the case if your BES and Mail server are not in the same vicinity e.g. in different countries.

Also see this article regarding the effects of latency on BlackBerry Enterprise Server for Microsoft Exchange.