rb600 performance

Hi All,
I’ve got a RB600 (latest 3.28 firmware) w/6 ubiquiti 5Ghz cards running to six different locations. Some issues are manifesting themselves as clients from 2 hops off this router report wildly varying ping times. Most of the time <20ms but have had periods in excess of 2 secs.
There are ultimately about 180 clients running traffic thru this board and it is in turn connected to a 7.5 M (5XT1) cisco gateway router. My question is , at the frequency these things run , why would I have ping times that would vary that much.
Is there a practical maximum number of radio data streams this thing can handle without choking? Clients report consistent pings to first hop but when taken to this board things start getting inconsistent. No nstreme is being used and link bandwidth seems to be adequate. Further , is it possible to bond these 6 wlans to balance traffic flows to get a more consistent flow thru the router? Six radios , six different subnets. Any suggestions?