Original: What causes RAID 5 to be slightly slower than RAID 10 on sustained reads is the fact that RAID 5 has parity data interleaved within normal data. For every read pass in RAID 5, there is a probability that a read head may need to traverse a region of parity data.
Simplified: The read performance of RAID 5 is almost as good as RAID 0 for the same number of disks.