What is Interfering SSIDs ?

Interfering SSIDs in this case means the number of neighbouring SSIDs/wifi networks at a certain signal strength (RSSI) that are on the same channel as your FortiAP's. This is commonly known as Co Channel Interference whereby multiple WiFi networks are using the same channel as each other. This means that clients connected to different AP's which are on the same channel are contending with each other for an opportunity to communicate on the wireless medium and slowness, sluggishness is what's commonly experienced in these cases. This is most commonly seen on the 2.4GHz network as there are only 3 reusable, non-overlapping channels. This can be seen on the 5GHz network too if you and/or neighbouring SSIDs are using 40/80MHz channel width.