Ok - time has passed by.
My kernel is "2.6.32-5-kirkwood" and I use dm-crypt/luks.
Testing an external USB Disk I get:
Timing buffered disk reads: 92 MB in 3.05 seconds = 30.15 MB/sec
testing the unecrypted partition on this disk.
Timing buffered disk reads: 24 MB in 3.26 seconds = 7.37 MB/sec
testing the mounted encrypted partition of this disk.
How do I find out whether hardware acceleration is in use or not?