Im using the same firmware version on an SZ300 and have 3 R700 AP's that I'm seeing the same behaviour with. High unexplained latency on 5Ghz see image below. Havent done a great deal of investigation yet.
We are running R710s with vsz100 controller 3.6.2.x.x, no vlanning and wireless clients connected to the APs are no where near 100, more like under 25 clients and 5g experiences latency. Would love to hear an update on your situation.