HDR playback laggy or impossible in QuickTime

> any suggestions or hints to help resolve this issue

Use IINA.

Try to completely remove Chrome, if it exists (at leas in Final Cut it is reported to cause poor performance).

https://chromeisbad.com

An EtreCheck report would obviate much guesswork:

Using EtreCheck to Troubleshoot Potential… - Apple Community

How to use the Add Text Feature When Post… - Apple Community

I had what I think is the same problem when playing .MOV video files from my iPhone on my Mac (Intel).

I just upgraded to Ventura (13.6.3). Upgraded my phone recently too (17.2.1). Both QuickTime Player and QuickLook became _very_ slow to load and play new iPhone videos. QuickLook seems to be caching them for multiple seconds, then they play normally. However, strangely, the Finder is slow/laggy when you _stop_ playing such videos for at least 5 seconds before the Finder returns to responsiveness. For example, can't move windows, turn volume with touchbar during these 5 seconds.

I found that videos recorded on the iPhone with "Camera->Record Video->HDR Video" turned off play fine.

(Confusingly, _sometimes_ playing the _same video_ (recorded with HDR on) is laggy, but sometimes not. I have not been able to isolate why. Even without rebooting in between tries.)

BTW the 5 second freeze even happens when doing Command-I "Info" on the file in the finder.

The difference in the Color Profile of 2 files:

HD (1-1-1) [plays ok]

BT.2020 HLG (9-18-9) [is laggy]

Also both play fine in VLC (probably uses different decoder).

Just wondering if you found a fix. I've been exporting my files through the Quicktime menu at 1080 and it fixes the issue but still a pretty annoying workaround.