Disk5 appears to be failing, you should run an extended SMART test, sync errors are likely the result of this:
Jan 14 10:03:01 Server emhttpd: unclean shutdown detected
As for the speed it could be disk related, but not disk5 since it's already past it, though parity is also showing some issues, but there also appears to be something else accessing the array, so stop all that first.