I know sometimes optical drives perform poorly when set as slave. I'm not sure of the reason for this however. While I'd like to suggest you put each drive as master on different IDE cables, Dell is using the latest and greatest from Intel, so you're likely limited to a single IDE port on the motherboard.

What I can suggest you check to make sure that your performance isn't being hindered by something else follow these steps:
-Right-click on 'My Computer' ---> Properties.
Go to the Hardware tab, click the button 'Device Manager'
-Expand (click the '+' sign) 'IDE ATA/ATAPI Controllers'
Right click on 'Primary IDE Channel'---> Properties.
-Go to the Advanced Settings tab. Make sure that both 'Device 0' and 'Device 1' (aka Master and Slave, respectively) are set to 'DMA if available'. If you have to change them, Windows may want you to reboot. Go ahead if it does.