Preview vs. Render time vastly different - Any ideas?

I have a fairly complex and large scene. I accept it’s going to be slow, especially with my system specs. I have been working on it in quarter res and it can easily preview render that.

But what I can’t get over is the render time!

A full quality preview takes 15-25 seconds to render a single frame. This is somewhere I would expect the final render to be. It doesn’t matter where I put the play head on the timeline it’s always between those times to preview a frame.

When I start rendering, it seems to be around this time, but then it starts getting really slow.

I have switched from ProRes to a PNG sequence to try and figure how long this is actually taking and where it’s slowing down and the results are...interesting.

At a point during the animation it goes from slowly getting longer per frame (each frame is taking up to a minute or so) to suddenly getting really long per frame, each frame is then taking around 15 minutes longer than the previous, and now I am up to over 2 hours per frame. There seems to be an exponential growth in render time at a certain point in the animation for no obvious reason. I could understand this might happen over the 20 minute scene, but not within the space of 10 frames (seconds to hours)! At 25fps, you can imagine the difference between one frame to the next can’t be THAT significant to change the render time that much.

I can absolutely accept a difference in preview vs. render times, but a 40,000% increase? And if I leave it, how much higher will it get?

I have disabled GPU acceleration to see if that helped, it didn’t. I have tried it both with the AE render queue and Media Encoder with no real difference.

I have tried it with or without DOF, with or without motion blur, with or without lighting. They all make a difference to the preview render time (around 5 seconds max), but don’t really change the actual render time.

As I said, it’s a complex scene, it’s got to be thousands of shapes being processed and a camera flying through them. I expect it to be heavy, what I can’t get past is the preview to render time differences.

Has anyone got an experience of this kind of problem?

FYI: Due to the scene, pre-rendering is next to impossible. The only thing I could do is multi-pass the scene with different passes and piece it together, but as the scene has elements happening around the camera and between each other, I would need to also render depth/opacity masks for each run and I am not sure if I can do that out of the box in AE. 


PC:
Source materials are on one NVMe, rendering to another NVMe.
CPU: 12900H
RAM: 32GB.
Latest AE (26)

EDIT: For clarity, this is a brand new project in AE 26 created in the last few days as a clean installation. It’s not using any 3rd party plug-ins.