Just recently the online game performance for Fifa 16 on PS3 has just gotten so ridiculous that the game is barely playable. Super bad response when playing, meaning that your almost guaranteed to lose every time.
I know there is a graph in the ASUS RT-AC68U but as to how quick it samples the speed is another question and if you don't have your QoS on or set correctly a burst of traffic for a device may not show as hitting your upto limit or is peaking higher then just gaming there is also how the ASUS RT-AC68U limits ping that may not be documented because you are likely allowing all IP's to ping you? Get enough of them and it can flood your upstream and that may not show on the graph in the ASUS RT-AC68U its not like I can know how every routers works and handles stuff. If you run this speedtest you ideally what to configurer it as best you can to show A http://www.dslreports.com/speedtest
If its still a problem you would have to really test by disabling the wireless to insure its none of them and with just the PS3 connected.