Why doesn't Multi-frame Rendering work with PNG sequences and only uses one core?

I did some research and testing and I think this is what's happening.

Rendering in After Effects consists of two main stages: Compositing and Encoding. Compositing involves calculating effects, transformations, and generating the individual frames. The Encoding stage is to compresses those frames into your selected format.

For formats that support asynchronous encoding or have extremely fast serial encoding, MFR is very noticeable. However, PNG only supports serial encoding and is also relatively slow at encoding. This means MFR applies only during the Compositing stage, but not during Encoding.

I've observed that the complexity of a Comp influences how much MFR benefits the rendering process for PNG. With more complex comps, MFR is more likely to be visible because Compositing takes longer, giving the encoder time to keep up. For simpler comps, Compositing finishes quickly, resulting in a queue of frames waiting for the slower PNG encoder to encode them one by one. This bottleneck effectively limits MFR and makes it behave like SFR.

The screenshot I shared earlier was from rendering a very complex 4K comp with 3D, lighting, and special effects. In this case, the Compositing stage was slow enough for the PNG encoder to keep pace. However, when I rendered a simpler comp with one solid, I experienced behavior similar to SFR, as you described.