The code that writes ZIL blocks uses dmu_tx_assign(TXG_WAIT) to assign
a transaction to a transaction group.  That seems to be logically
incorrect as writing of the ZIL block does not introduce any new dirty
data.  Also, when there is a lot of dirty data, the call can introduce
significant delays into the ZIL commit path, thus affecting all
synchronous writes. Additionally, ARC throttling may affect the ZIL
writing.