FastLeaderElection has final static int finalizeWait = 200. This is time to wait after successful leader election. I don't know what could happen, but 200ms is too slow for production environment under heavy load.
I changed it to 20ms and everything still works for me.
I propose to make this value configurable with default value of 200 to not affect current installations.
Combined with #ZOOKEEPER-1515 it could improve leader election and make it 10x times faster: 1500ms -> 180ms including 100ms for 2 faileed new leader connections.