Improving disk mirroring

Yes. Task performance tuning can also be used for mirror resync.

Have a look at this recent discussion:
 https://www-secure.symantec.com/connect/forums/adding-mirror-too-long#comment-10358801

Have you enabled Dirty Region Logging on the mirrors?
It will speed up resync and only sync the regions of the volume that are marked as dirty in the dirty region log.

Please note that 'day to disk mirroring' does not do a 'resync'. 
It simply writes to each copy of the mirror simultaneously.
Mirror resync is only required when mirrors are no longer in sync, for example when there are outstanding writes when the system craches. 

The SFW Admin Guide has a Troubleshooting section.
See:  https://sort.symantec.com/public/documents/sfha/6.1/windows/productguides/html/sf_admin/
 

You can find SFW manuals here:  https://sort.symantec.com/documents/doc_details/sfha/6.1/Windows/ProductGuides/ 

Manuals for other versions:  https://sort.symantec.com/documents