I would say that these two messages:
dhcp lease/renew/rebind -1/0/0
DHCP after tweak, dhcp lease/renew/rebind -1/-1/-61
might have something to do with your problem. I'm not exactly sure what values are supposed to do what here, but my empeg recieves MUCH bigger values here. Yours things that it has to renew it's IP lease with the DHCP server at VERY SHORT intervals. That would definitely slow the server down.

Can't tell you whose fault it is though. Either the server is sending the wrong values, or the empeg is interpreting them incorrectly.