Benefits

You can gain elapsed time savings by using cache fast write. The benefits of CFW are twofold:

When writing data to the work data sets, the write operations can complete at cache speed rather than at disk speed, as long as overall cache activity permits it. If the cache usage is very heavy, then there may be very little or no benefit to using CFW, since the majority of write operations will be immediately directed to disk.
When reading data back from the work data sets, the read operations can complete at cache speed rather than at disk speed, as long as the data to be read still resides in cache. If the required data does not reside in cache, a disk access is required and no performance benefit results. The higher the cache activity, the less the performance improvement with CFW, because it is more likely that written data will be destaged to disk before it can be read back.