Software Raid question

["mike" <mike@heri.sd57.bc.ca>]

morning,

I have a software Raid 1 setup with 2 disks.
/dev/hdc1
/dev/hdd1

I've noticed that preformance has slowed a bit since the raid setup.
I also have both disks on the same controller. 
Would it make a difference if the disks were on seperate controllers?
ex.
Move /dev/hdd1 to /dev/hda1, therefore raid would be
/dev/hda1
/dev/hdc1

If this does indeed help performance, is it safe to just edit the 
raidtab file and reboot? or will the raid array re-sync and wipe the data?

Thanks