The Newifi has more than enough power to handle your network speed. There shouldn’t be any performance degradation even if you enable all controls.
Adblock and SafeSearch are both DNS level filters, there’s no overhead over normal DNS lookup. Also, did you mean there was no Internet at all when you enforce Access Control? (that shouldn’t happen)
As for WiFi, you may want to use different SSIDs for 5GHz and 2.4GHz, so that you know which band the devices are connecting to. For old 2.4GHz devices, you might want to change the channel width to 20MHz (on the router Wireless Settings page) to see if it helps. WiFi should perform better than the TORONTO-N without tinkering with QoS etc.