Unraid does not rebuild parity when you add new disks to a parity protected array - instead it Clears them (sets them to be all zeroes) so that parity is unaffected and only allows you to format and start using the drives once they have been Cleared. If they have been pre-cleared then this Clear stage is skipped and they are immediately available to be formatted and used when added.
The Unraid Clear is functionally equivalent to the write phase of a pre-clear and takes about the same time as that would take, but is not as thorough a test of the drive. Normally with pre-clear one tends to run some read phases as well to check the disk better which is why pre-clear typically takes longer.
Preclear is normally slower if you run the read phases as well as the write phase so no idea where you heard this? Perhaps that running multiple pre-clears in parallel can put more stress on the server?