Apple TV WiFi is very slow
Why is my AppleTV WiFi slow
Apple TV 4K, tvOS 14
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Possibly you experience interference on your home network, causing overload on the remaining diminished bandwidth.
Restarting your router may automatically select new optimized Wi-Fi channels with less interference from neighboring networks, if set to automatically select the best channels (once at boot time). Or manually find channels for both bands in a relatively empty zone for your router configuration.
Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
If your Apple TV can't connect to Wi-Fi - Apple Support
Recommended settings for Wi-Fi routers and access points - Apple Support