A single SATA disk can do about 75 IOPS and a single FC disk 150 IOPS.
For random type IO, these numbers matter and you need twice the number of SATA spindles compared to FC disks to satisfy a certain random IO load. For sequential IO, there's much less of a difference because SATA disks can handle sequential IO quite well.
This is the reason why FC/SAS disks are recommended for apps like databases that generate lots of random IO. File storage (especially for large files) is more likely to be sequential IO and SATA will be ok for this.
All of this is true for medium work loads. However, if the work load is much too high for the aggregate to handle, we have the impression that FC disks can handle the "overload" better than SATA disks, providing a kind of "graceful performance degradation".