I can suggest that it will generally be a slow process as RAID5 disks are optimised for more reading than writing [since the parity calculation and then striping of parity across multiple platters take time]

Just to be pedantic, you forgot the fact that the system also has to read in other blocks when writing. That is, the interleave size of RAID5 is usually larger than a disk block, so when a disk block is updated, the system first has to read in the rest of the interleave set before it can even calculate the correct parity. I hear some more intelligent ones will read in the related parity and calculate it off of the old and new blocks instead of the entire set, but AFAIK, that's fairly uncommon, and it still requires yet another read. Regardless, you're right; RAID5 is slow to write.