Unlike hardware RAID controllers, SoftRAID can perform many RAID operations at exactly the same time.
In addition, the RAID engine keeps track of not only which disks are out of sync in a volume, but also which parts of those disks need rebuilding. If you only write to the first 1% of a volume while a disk is disconnected, then only that 1% will be rebuilt when the disk gets reattached. This dramatically reduces volume rebuild times; a 16 TB volume can usually be rebuilt in under 10 minutes (with 10 – 30 GB of changes).