Too many APs and too many SSIDs will cause performance issues if not tuned correctly. It does not matter who the vendor is Extreme, WiNG, Cisco, Aruba, Aerohive... If your wireless network is not configured correctly, there will be problems. The best real advise is to work with your local Extreme WiNG SE to assist in tuning your environment to provide the best experience for your customers.
When running that many SSIDs on a single AP (x53) the management traffic alone will kill performance. Several things to consider. Lowest data rates. Set them to only connect at nothing lower than 9MB, best would be 18 or 24MB data rate. This will force all to connect at higher rates, this leaving ample room for traffic. AP management traffic will also use the higher rates to pass traffice to the RF-Domain. This is good for the 2.4GHz radio. For those on 5GHz, you should be okay to leave at default.
When you do not broadcast the SSID, each AP has to ask every AP what they have to offer. Again, this creates more non data overhead. Best to go ahead and broadcast the SSID. Hacker can find even if not broadcast if that is what you are worried about. Just be sure to enable strong security measures on each.
To help reduce the number of SSIDs, try using dynamic vlan assignments. This works with your AAA policy and RADIUS server. This helped us out greatly to eliminate 4 SSIDs so far. When issuing a "show wireless clients", add the column to also include the vlan. Guest or freewifi, should still have its own SSID and routed directly out. Other Corporate SSIDs, should roll into 1. Legacy devices may need their own based on what type of authentication functions they can handle.
You will need to research each device type and their wireless capabilities to determine basic lowest rates (work to raise up to 24MB).
Research each device authentication capabilities to eliminate SSIDs and use dynamic vlan assignments using AAA policy and RADIUS.