Hard disk performance drastically declined after OS X Lion upgrade
Ok, I bought a new Harddisk (Samsung 640GB) and cloned the old HD with SuperDuper. After Spotlight has re-indexed the drive I have the following benchmarks:

Sequential Write: 60  MB/s, 25 MB/s
Sequential Read:  17 MB/s, 69 MB/s
Random Write:  1.5 MB/s, 24 MB/s
Random Read:   0.5 MB/s, 19 MB/s


I don't know if the HD or it's controller is damaged, but now the performance of my system is ok again!