A few weeks ago I had a workload which I suspected had a fair amount of synchronous writes on a ZFS file system. The general recommendation for this case is to use a separate ZIL log, preferably on a device which has a nonvolatile write cache or a write-optimized SSD. However, once you add a separate log to a pool, you cannot easily remove it. So before you make that move, it is a better idea to look at your workload and answer the question, "how much ZIL write activity is generated by my workload?"
The result of this pondering is a script I call zilstat. You can get it here.