Serial Console and Slow SCSI Disk Access?

[Jeff Gold <jgold@mazunetworks.com>]

Does it make sense that enabling a serial console would reduce the 
bandwidth of a SCSI disk by more than a factor of ten?  This seems crazy 
to me but there's no arguing with empirical facts.  I have an IBM x345 
system on which I installed an unmodified 2.6.16.20 kernel built with 
gcc-4.0.2-8.fc4 (the kernel configuration file I used can be found at 
http://augart.com/jgold/kconfig).  With the control kernel command line 
I get about 70 MB/sec with hdparm -t but when I add "console=ttyS0,9600 
console=tty0" I get about 1.6 MB/sec instead.

Here are the complete kernel command lines from grub.conf (the second 
one doesn't actually include a backslash in grub.conf -- that's just to 
avoid email wrapping):

   ro root=/dev/sda3 pci=biosirq loglevel=7

   ro root=/dev/sda3 pci=biosirq console=ttyS0,9600n8 console=tty0 \
     loglevel=7

I have not been able to connect this behavior to any known kernel bugs 
using Google or mailing list archives.  What might I be doing wrong?

                                          Jeff