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I'm rendering an animation in Cycles and the animation is getting really slow at the last frame. I checked all of my settings and animation keyframes and confirmed that the settings were identical throughout the render. The render is fairly heavy, but keeps the same amount of objects and detail throughout the animation.

Render times

The 149th frame (the last frame) is taking about 60 times as long to render. This doesn't bother me that much right now, but I will need to render much more heavy scenes in the future, and would like to know why this is happening.

If it helps, here are my system specs:

  • RTX 4080 GPU, i7 13700kf, and 32GB of RAM.

I am rendering with Optix. I also am saving as an FFmpeg video, so I can't just quit the render on the last frame.

Update: There was one more frame (frame 150) and this one is taking an absurdly long time to render. The time estimate shows 8 hours, which I can't afford to wait for. Canceling the render causes the GPU to go crazy, and I have to force quit the application. I reopened Blender, and tried rendering the animation again. The same thing happened. I also tried rendering the frame in question by itself, and it took less than thirty seconds.

Update 2: On the third render of the animation, it becomes super slow from frame one, frame one being estimated to render for 18 hours. Upon further investigation, it seems like a particle emitter in my scene is the problem. I tried reverting to a version without the emitter, and it rendered fine. I also tried simply pointing the camera away from the scene and towards the HDRI I was using, and it also rendered as expected.

However, I need the particle effects that are causing the problem, and I also don't know why the particle effects aren't rendering. When I render an image with even 10 times the particles, it still manages to render in under 2 minutes, yet each frame in the animation is estimated to take days.

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I seem to have fixed the issue by slightly moving the camera. I have no idea why this worked, but the frames now seem to render normally.

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