[CentOS] variable raid1 rebuild speed?

I have a 750Gb 3-member software raid1 where 2 partitions are always 
present and the third is regularly rotated and re-synced (SATA disks in 
hot-swap bays).  The timing of the resync seems to be extremely variable 
recently, taking anywhere from 3 to 10 hours even if the partition is 
unmounted and the drives aren't doing anything else and regardless of 
what I echo into /proc/sys/dev/raid/speed_limit_min or _max.  Is there 
some way to tell if the drives are going bad or speed it up consistently 
if they aren't?
-- 
   Les Mikesell
    lesmikesell at gmail.com