4K Video doing jerky direct play on 720 tv

[WoodyBABL]

Just future - proofing a YouTube download, so I grabbed a 4K version. I'm at a relative's place using a Chromecast with Google TV (the white dongle) on a 720 TV. 

When I select the 4K video, the dashboard says it's fully compatible and doing direct play. The video does show up but is jerky and so unwatchable. Any idea why it's misbehaving. What should I be looking for? 

Video
Title: 4K AV1 SDR
Codec: AV1
AVC: No
Profile: Main
Level: 12
Resolution: 3840x2160
Aspect ratio: 16:9
Anamorphic: No
Interlaced: No
Framerate: 25
Bitrate: 11776 kbps
Bit depth: 8 bit
Video range: SDR
Video range type: SDR
Color space: bt709
Color transfer: bt709
Color primaries: bt709
Pixel format: yuv420p
Ref frames: 1

Audio
Title: English - OPUS - Stereo - Default
Language: eng
Codec: OPUS
AVC: No
Layout: stereo
Channels: 2 ch
Sample rate: 48000 Hz
Default: Yes
Forced: No
External: No

[crobibero]

The chromecast client is fairly limited so it must not recognize that the tv it is plugged into doesn’t actually support 4k playback, so when it tries to play the movie it is attempting to convert from 4k to 720p causing the jerky playback.