ZIL is only used on synchronous IO. Moving it to a dedicated SLOG device would have no impact on non-synchronous IO. A SLOG device does help on synchronous IO though.<p>That said, all file systems degrade in performance as they fill. I do not think there is anything notable about how ZFS degrades. The most that I have heard happen is a factor of 2 sequential read performance decrease on a system where all files were written by bit torrent and the pool had reached 90% full.  That used mechanical disks. A factor of 2 in a nightmare scenario is not that terrible.