@nicOo110 

Hi, sorry for the delay.

" If I turn on the guest network or IoT network, will it reduce the performance of the main network if the guest network isn't using any bandwidth?"

Usually, it would not reduce the performance. But the same bands on all WiFi networks broadcast by Deco use the same channel. For example, the 2.4GHz band of the Main WIFI and the Guest WiFi would all use channel 11, which might cause internal WiFi interference. However, the impact of this interference is expected to be minimal.

"And if I want to connect the sketchy CCTV to the main network, will turning on device isolation on the device in the Deco app have the same effects as a guest/IoT network?"

You can refer to Q&As here: https://community.tp-link.com/en/home/stories/detail/501928

The devices under the "Device Isolation" group can talk to each other, but not to the devices outside of the isolated group. While the Guest network devices can't talk to each other.

Thank you very much.

Best regards.