Adding a disk into a volume will cause increased CPU and I/O load for < 10 minutes while data is copied over.
"Copied over"? When you add a disk to a volume, we zero it (so that adding it to a RAID group doesn't change the parity value for the stripes it joins); we don't copy data (in the sense of data written to the file system on behalf of clients) to it.
That will (if the disk isn't one that supports the SCSI "WRITE SAME" command, or that supports it but not reliably) consume some CPU time and I/O bandwidth writing to the disk. If the disk does support WRITE SAME, it requires less CPU and I/O bandwidth (a chunk of zeroes should get transmitted to the disk once, and then the disk should write that chunk of zeros to a large block of sectors - we don't zero the entire disk in one command, in part so that, as I remember, we can give a progress indication, and for other reasons I no longer remember).