Each SSID broadcast by an AP adds additional management frame overhead, such as beacon frames. These beacon frames are transmitted periodically by the AP at the lowest mandatory data rate (e.g., 1 or 6 Mbps), which consumes airtime that would otherwise be used for client data.

According to CWDP-305:

“Each SSID increases beacon overhead. More SSIDs lead to greater airtime consumption by management traffic, which in turn decreases available throughput for actual client data traffic.”

Reducing the number of SSIDs is a best practice to minimize management overhead and optimize throughput in the wireless network.

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on Infrastructure Design