Slow render speed on MacBook Pro M4

First we must examine the overall pixel export rate, not the frame export rate. It is pixel export rate that defines the amount of work:

3840 x 2160 x 63 fps = 522,547,200 pixels per sec

1920 * 1080 x 240 fps = 497,664,000 pixels per sec

1280 * 720 * 320 fps = 294,912,000 pixels per sec

So it's actually processing more total pixels per sec in the 4k case.

4k has about 400% of the pixels of a 1080p frame and, at the same frame rate, requires about 400% of the computation, whether that is handled by software or hardware.

For the 4k export case, it's likely the hardware encoder handles most of that, thus the low power consumption. 522 million pixels per sec might be the single-encoder limit for that case. Since the encoder is specialized hardware, it does most of the work, not the CPU threads or the GPU. Hence power consumption is low.

The M1/M2/M3/M4 Max have two encoders and the M1/M2 Ultra and upcoming M4 Ultra have four encoders. Using FCP's segmented encoding feature, those will export faster even on a single output file.

But your M4 is probably already faster than most other machines at exporting.

For the 1080p case the export pixel rate is a little lower than at 4k, but it apparently takes more power. That might be due to CPU or GPU activity required to downscale the material to 1080p. Encoders don't scale, just encode. So something in the pipeline must downscale that before submitting to the encoder. That could have a power cost.

For the 720p case, the pixel export rate is considerably slower than 4k and 1080p, and power consumption is higher. Like the 1080p case, that could be due to CPU/GPU computational cost of scaling before handing to the encoder. The export pipeline may have maxed out the available CPU/GPU resources to downscale the material before handing to the encoder, which could explain the even higher power consumption and the lower pixel export rate.

Suggestions: Don't use multi-pass H.264. That is not needed, and there is virtually no quality difference.

Try single-pass H.264 vs HEVC (H.265) and examine the relative encoding speeds on your machine.

• Create a new test library on both M2 Air and M4 Pro.
• In FCP Settings>Playback, turn OFF background render on both machines.
• In FCP Settings>Import, select "Leave files in place" on both machines.
• Put a single single Osmo 4k/60 clip on the internal SSD of each machine.
• Import the clip to the new test library on each machine. Do not put it on a timeline.
• On each machine, select the clip in the Event Browser and do File>Share>Export File, format: Computer, Video Codec: H.264 single pass, resolution: 4k.
• Pick as export location the internal SSD.
• Measure the export time on both machines.

The export time gives a value for the export and encoding performance of both machines, separate from effects and render cache state. Depending on these numbers, we can decide what to do next.

I have an M4 Pro MacBook Pro, so probably a very similar machine to yours.

What are your export settings, exactly?

Some exports may take advantage of the built-in hardware encoders, while others may not.

By the way, using more power does not necessarily translate to working faster.

You may want to look at what Activity Monitor says.

If the export is done mostly in the hardware encoders, then you'll see low CPU utilization - and that is a good thing.

I tried a few exports. I took a timeline of about 18 minutes and timed export to 1080p and 4K at 60fps. I exported to HEVC at 10 bit. Export at 1080 took some 5.5 minutes, and export at 4K to about 18 minutes.

I also tried similar exports at 25fps and, as expected, they took significantly less time.

I then tried a different thing: I took the exported 4K 60fps file, and tried encoding it using HandBrake. This one uses the CPU, and pushes it to around 1000%, so many cores fully used. Lots of power (and in fact this is the first time I'm hearing this Mac fans working!).

But guess what: it is doing so at about 28fps, so despite all that energy use, it cannot match the dedicated hardware encoder.

HEVC is a very compute intensive codec, and 4K at 60fps is a lot of pixels.