Poor SSD Raid1 performance

Hi all,i know, software raid is not supported but maybe someone has an idea on how to solve this:
I have a fresh install of prxmox on a HP Proliant ML110 G7 with data (LVM) on a Raid1 located on two SSD Samsung 840Pro 512G.
Everything seemed fine after creating md0 on /dev/sdb and /dev/sdd but after syncronisation finished performance is very poor.
I get io waits up to 80%.

atop shows that one of the disks is very slow, the other one looks good
DSK | sdd | busy 81% | read 36 | write 384 | KiB/r 29 | KiB/w 128 | MBr/s 0.10 | MBw/s 4.81 | avq 2.79 | avio 22.4 ms |
DSK | sdb | busy 17% | read 915 | write 381 | KiB/r 41 | KiB/w 129 | MBr/s 3.68 | MBw/s 4.81 | avq 3.24 | avio 1.49 ms |

DSK | sdd | busy 96% | read 13 | write 158 | KiB/r 8 | KiB/w 167 | MBr/s 0.01 | MBw/s 2.58 | avq 6.42 | avio 58.4 ms |
DSK | sdb | busy 7% | read 74 | write 164 | KiB/r 18 | KiB/w 164 | MBr/s 0.13 | MBw/s 2.64 | avq 3.97 | avio 3.18 ms |

So one of the disks is about 10 times slower than the other.Any idea?

Best regards, Holger