How does your traffic look? Is the network getting pounded by some large amount of traffic. If the firewall has to discard a lot of packets that could be eating up all the CPU.

Also, is the empeg set for dhcp or static? It sounds like there is some broadcast here that is killing the firewall. Empload can also search for the empeg via ethernet broadcast.

Lets narrow it down a bit more.
The empeg only has to be plugged in? plugged in and turned on? plugged in, turned on and empload running?

when exactly does the slow down occur?