A RAID rebuild is the data reconstruction process that occurs when a hard disk drive has degraded and needs to be replaced.
When a disk fails unexpectedly, a RAID array will copy data to a spare drive while the failed one is replaced.
Data will reassemble on the new drive using RAID.
RAID rebuilds can also be proactive if the RAID array detects that a hard drive is about to fail.
During the rebuild, the performance of some applications or processes will be delay, and will result in slow performance for the server.
For more details about different RAID level, may refer to below link :