in your WinPE image, if possible. The default to take all drivers means the boot process has to indentify the NIC for the machine being imaged, initialzie it, and then obtain a DHCP address. That all takes time. There was also a process to cut down the size of the WinPE boot image in general, not sure if this is still valid for WinPE 2.1.
Since we're taking a different road in the forest with this line of the questioning, did the above links help out at all, in regards to your initial question?