Since a RAID 5 array is not very different from a RAID 0 when it comes to read operations, it offers high performance due to the use of striping. Writes, however, take considerably more time, since the parity generation step is involved for each stripe set that needs to be written. As a consequence, read performance scales nicely with any drive you add, but write performance is mostly somewhat limited.
From a performance point of view, RAID 6 should not be very different from a RAID 5. In practice, however, it usually is; this is due to varying implementations of the second parity layer. Regardless, write performance decreases even further due to the increased XOR load.
Isn't RAID 5 Redundant? Why RAID 6?
The main reason for setting up a RAID 5 is that data safety is increased while maintaining acceptable performance levels. Yet the safety turns into a real threat as soon as one array drive member fails: A degraded RAID 5 array is vulnerable to any additional drive loss, since once the first drive fails it is effectively a RAID 0 array! Unfortunately, the probability of losing more than one drive increases with the number of drives, so if you want to do it right, your first priority is to ensure that the array is always protected. You should always provide a so-called "hot spare" drive that can automatically be used to rebuild the array in case any array drive dies.
People sometimes come to the premature conclusion that a degraded RAID would not be different from a single hard drive in terms of data safety. As a matter of fact it is worse! While there is always a certain chance that a drive could fail, the chance of a degraded array failing is a function of the total number of hard drives within the array.
RAID 6 looks like the perfect way to reduce the chance of disaster, because it allows for two drive failures without data loss. Obviously, this scenario is less likely that losing only one array drive, which makes RAID 6 the more secure storage configuration.
Due to the double redundancy, RAID 6 is not quite suitable for environments with high performance requirements. Some people may also consider as an alternative a nested RAID setup such as RAID 5+1 (two RAID 5 setups mirrored). It must be noted, however, that these require a large number of hard drives (a minimum of six) and like RAID 1, the total size of the array is only half of the sum of the drives' capacities.