Very poor playback performance for 4K video, while VLC is fast

VLC is using hardware-accellerated decoding and display (the decoded, uncompressed video stays in GPU memory). Shotcut does not because it is primarily a video editor. You can read more in the FAQ linked at the top of this page. The Shotcut remedy for this is Proxy and Preview Scaling.

On my modest system (Core i7-9700K), the clip you shared only required ~27% CPU to play in the Source player when using Video Mode Automatic. On the Timeline 28%. If your Video Mode does not match the clip’s attributes, Shotcut must scale it and sometimes do color space conversion. The performance will depend on Settings > Interpolation and Settings > Realtime. With 1080p60, realtime on, and bilinear interpolation, my system uses ~39%. With realtime off and lanczos interpolation my system uses ~61%. Another big factor is Settings > Display Method. If add software display method to the previous test, now I am using ~85%.

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