It must be a local problem on my iSCSI box. Now i restarted the restore and during this i tested again with IOMeter from my Workstation. The value for read is now between 0 and 5 MB/s...
The workstation is connected to a Intel GigaBit Server Adapter which is placed in a PCI-X slot. The ESX Hosts are connected to the both on board Intel GigaBit NIC's.
Can i made some other tests? Where is the bottleneck?