geom mirror write performance problem

[OxY]

i found the real problem, it was not the xl driver, but the geom mirror..
if i try to write to an other disk, not in the main systems raid1, then
download is ok, 10M/s...
then i tried to remove one disk from the raid1, and write on it..
the download is OK again...
when i sync the raid array it gives poor performance again 1M/s..
what's wrong?

the disks are OK, all hardwares too, it worked for almost a year without 
problem..
i upgraded from 5.4->6.0-rel 1week, that's when the problem appeared..

i use geom mirror on 2 other servers (5.4, 6.0) and works fine, fast 
enough..
any idea what to do?