This makes me wonder if you know that parity only needs to be as large as the largest single data disk. Parity is not a backup and contains no data, but it looks like you may have sized parity so it is as large as the total data storage. 8TB of that parity is currently wasted, but it does mean you can use 12TB data disks in the future.
Once parity check hits 4TB it isn't doing anything at all except verifying the rest of parity disk is zeros. In fact other disks can spin down at that point unless something else is using them.
Doesn't explain the slower progress at that point though.
Syslog seems to indicate parity is being corrected, probably beyond the point where no other disks are involved and parity should be all zeros already.
Are you sure you did the initial parity build when you added that parity disk? What else can you tell us about how you got to this point?