sorry, I'm pretty inexperienced with unraid but what I probably should have said was don't use a cache drive on a share where you store vdisks that are larger than the cache drive and you actually do use parity drives in raid 5 and 6 and they are usually the main limiting factor on writes.
In RAID5 the parity is stripped accross all the disks in the array. So a file you write is written simultanteoulsy to all drives in the array. In effect the filesystem is split over a multitude of disks, where data to recover from a failed disk (parity info) is also written against all drives.
In unraid all disks have their own filesytem and parity is maintained on a seperate disk. (or, if you want to protect against dual drive failure) on two seperate disks. .
Wrt to the comment on vdisks and array and cache drive size: a vdisk you would not store in the array (and use a cache drive for), you would probably have it sit on the cache drive also for reasons of speed (or, if speed is not a thing, you would have it on the array constantly). The VDISK would never move back to the cache drive (that mechanism is only used when ADDING files, not when changing them).
edit: it turns out I was wrong and rushed to an incorrect conclusion and misread the diagram https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5 striping the parity wouldn't work.