Simple BDE disc encryption benchmark

Hi! I just ran a very simple benchmark on the GBDE disc encryption in
CURRENT. The benchmark setup looked like this:

* Slow machine (Celeron 366, 128MB mem)
* 5-CURRENT from yesterday, running off of some old ATA disc
* 2 x 9GB 10k rpm SCSI discs hooked up to an Adaptec 2940

The benchmark was to copy the /usr directory (copied from the ATA disc,
1.7GB) or a directory containing big files (/bigfiles, 1.7GB in 16 files
created by 'dd if=/dev/random ...') from scsi disc 1 to scsi disc 2. I ran
each benchmark twice and took a simple average of the results.

unencrypted to unencrypted:
/usr     :  697 real        10.6 user       235 sys  (~50% idle)
/bigfiles:  123 real         0.4 user        84 sys  (~25% idle)

unencrypted to encrypted:
/usr     : 1778 real        10.7 user       236 sys  (~35% idle)
/bigfiles:  379 real         0.4 user        82 sys  (~10% idle)

encrypted to encrypted:
/usr     : 1978 real        11.6 user       242 sys  (~25% idle)
/bigfiles:  615 real         0.4 user        80 sys  (0% idle)

The only time the CPU was completely busy was when copying /bigfiles from
encrypted to encrypted.

My question is: Why does the it take so much longer when encryption is
involved even though 'top' seems to think there are CPU cycles left to burn?

/Daniel Eriksson