inconsistent parity check speeds

[Smitty2k1]

I've noticed that my parity check speeds are usually consistent at about 121 MB/s, however I occasionally have a parity check that goes MUCH faster, over 200 MB/s. Any idea what causes the inconsistencies? Or maybe a better way to ask it - how can I speed up my parity checks? See attached image for history.

 

 

[JorgeB]

Post diags when its's going slow in case there's something visible, and to make sure the faster ones are correct, not many disks can give that as an average speed, could be wrong.