Exchange Server Latency opening emails

We have installed a new powerhouse Exchange 5.5 server in our site. LOTs of memory, disk etc. All the users we have moved to this new server are experiencing a “lag” when they try to access emails with attachments (even if the attachment is simplya link or something extremely small).’
If we move the mailbox back to an older server they do not experince this problem.

THOUGHTS???? Help please!!!