On the original system I have :

# /sbin/sysctl dev.raid.speed_limit_min
dev.raid.speed_limit_min = 1000
# /sbin/sysctl dev.raid.speed_limit_max
dev.raid.speed_limit_max = 200000

After the suggested change the sync starts flying and the system is
still very responsive. Thanks a lot Anton, looks like the default is
much too conservative for my system!

BTW since I'll be doing some pvmoves later on does a similar tunable
exist to speed it up too ? (will move the LVM which is on this raid to
another software raid array)

Regards,

-- 
Nicolas Mailhot