Freebsd 5.2.1 Performance Woes

[TM4525 at aol.com]

While I was had a nice little test set up, I figured I'd test Freebsd 4.9 
against 5.2.1 since I had fresh installs handy on separate drives.
The simple test was as follows:

Hardware:

Celeron 2.4Ghz processor
Dual onboard Intel (em) NICs, 32bit, 33Mhz bus

Setup:

Traffic Generator -> FreeBSD System -> Server

I had it set up to route between the 2 interfaces (ie NOT bridged). I
fired a unidirectional stream of UDP packets (so there was no return traffic
from the server, which was discarding the packets) at a rate of ~100Kpps. 
The disturbing results:

FreeBSD 4.9: 33% interrupt usage
FreeBSD 5.2.1: >80% interrupt usage

What has happened to the stack?