How slow is "very slow" ? Is this different from the update rate you have configured when you set up the DDE/OPC driver ?
Are you using RSLinx Classic ? What Windows operating system and version of Excel ? What version of ControlLogix firmware, 1756 Ethernet module firmware, and version of RSLInx Classic are you running ?
How many elements are you reading from the controller ?
Is this an application that worked correctly in the past ? If so, what has changed about the computer or the controller ?