A 4K MP4 from my camera plays smoothly in VLC, yet the moment I put it on an editing timeline, scrubbing becomes jerky and preview drops frames even before I add effects. I assumed that if a player can decode it, an editor should have an easier job. Is this a sign the file is damaged, or is timeline playback fundamentally more demanding?
A player mostly decodes forward in order. An NLE has to seek, decode around cuts, maintain audio sync, generate thumbnails and prepare frames for effects. Long-GOP H.264 or HEVC therefore can be easy to watch but expensive to edit.
Codecs & export
Inspect the stream inside the MP4: codec, profile, bit depth, frame rate and chroma format. The .mp4 extension is only the container. A 10-bit HEVC clip and an 8-bit H.264 clip can behave very differently.
Before transcoding the originals, test hardware decode and then a proxy. If the GPU video-decode engine is idle while the CPU spikes, the application may be falling back to software decoding.
That clears up my main misconception: I was treating MP4 as the codec. I will inspect the stream and test hardware decode/proxies before making any new master files.
One detail that helped me diagnose a similar case was scrubbing with every effect bypassed and then watching the GPU's Video Decode graph. The overall GPU percentage was low, but the dedicated decode engine told the real story.
I checked it. The clip is HEVC Main10, and this laptop is decoding it on the CPU. A low-res proxy is smooth immediately. So the file was fine; my assumption that 'small MP4 = easy MP4' was the bad part.
That result is a good reason not to transcode the archive blindly. Keep the HEVC original, use the proxy for editorial work, and only change the master format if there is an actual compatibility reason later.
Editing workflows