TVs cant instantly switch because of the syncing of audio and video, all video signals are heavily processed in all modern TVs, Audio is generally decoded. to Match this delay of recording vs processing TVs need a few ms to buffer process and sync. This will always be the case using HDMI as the standard offers no advance warning just a new data rate / resolution.

The speed of the change of TV is dependent on the TV, not the device supplying signal. TVs dont prioritize the switching and there is loads going on to match audio and video. HDMI standard supports many resolutions, if in the AppleTV select match resolution then it will convert so the TV will not have to change.

Biggest issue for any “smart” tv is long term support and crap apps.

I have 3 smart TVs , 2 of them no longer get apps nor updates.

I have appleTVs on all my TVs, just works.