You cannot change the interval that the BES checks the mailbox. It's hard coded functionality and the Policies do not allow this.
99.999999999% of the time the BEs does not look for email either, it's the Exchange server that tells the BES that there is mail for a user. Only then does the BES get the email. That's why it functions like Outlook, Outlook gets notified the same way as the BES.
If you mean the latency is that the handheld gets the email before Outlook then it's not latency and it's not caused by the BES.
Less then 300 users should not be causing any latency issues like that, unless they have mailboxes outside of the recommended size limit. People with large mailboxes, in excess of 2GB, always end up being the cause of issues. This goes both ways for BES and Exchange.
Maybe this function is also available in the BB policies? However I would prefer to do not use a policy for this but set it on the BES itself.
BB Policies are on the BES. Not sure if you mean a setting on the individual handheld. You will not get the same functionality from your WM device and your BB, and for good reason. BB in my experience outperforms WM devices and the compression on BB is far greater then WM. WM data packets can be up to 143% larger in size then a BB data packet. That means more network traffic and higher data costs. I'll stop here about WM devices, they're not my most loved device.
What kind of latency are you expoeriencing and what leads you to beleive it is the BES?
Are the BES and Exchange on separate machines?
What specs do the machines have?