Multi-frame rendering slower with Keylight than single frame

I've just been trying out multi-frame rendering with a small sequence that has a single instance of Keylight 1.2 on some video footage.

I initially tried it just RAM previewing and found that the time to render was the same for single and multi-frame rendering. I could see that multi-frame was working and CPU utilisation was much more. Info said that at some points, 8 frames were rendering,

So then I tried a proper output from the AE render queue. I tried both single and multi-frame. The single took 31s and then the multi-frame took 54s! I had purged memory and disk cache between each attempt. I was saving as ProRes 422 in Qucktime wrapper.

Specs - Windows 10 Pro 10.0.19043

Intel i7-7700K 4.2GHz 4 core

ASUS Maximus IX hero

64GB RAM

NVME system and data drives

AE 22.1.0 (build 1)

Happy to share project comps if needed.

Paul.