I found out why my net4501 was going slow: the latest bios had the cpu cache disabled. :-) Its so nice when you finally get to the bottom of something.
To update the bios, I had to use linux. Even though Mac OS X runs BSD, I couldn't get the bios to update using minicom or lrzsz. :-(
I used linux, connected a null modem cable, and then booted up minicom and the net4501.
comBIOS ver. 1.31 20070408
comBIOS ver. 1.31b 20070614
Is it faster? YES! It is WAY faster. It can load the system.php page in under 5 seconds, whereas previously it required 10-60+ seconds, depending on the configuration.
Would have been nice to get a notice from Soekris about this, but as imply on their website, customer service is not their forte. :-)