stabilityai/stable-diffusion-3.5-large · Drops to single thread for a long time after iterations

I run StableDiffusion3Pipeline from a checkout of stable-diffusion-3.5-large at commit ceddf0a7 with num_inference_steps=80 on CPU, not GPU. It loads the checkpoint shards and pipeline components then runs through the 80 iterations on a few CPU cores. However, after the 80 iterations, it then drops to a single core for a very long time before actually writing the image.

I don't see the same from FluxPipeline: that runs through the 80 iterations on a few cores then promptly writes the image (pipeline.images[0].save).

Any idea why stable diffusion is adding this extra single-core phase after the iterations are complete, and if there's anything I can do about that?

accelerate==1.4.0 diffusers==0.32.2 huggingface-hub==0.29.1 mpmath==1.3.0 numpy==2.2.3 peft==0.14.0 protobuf==5.29.3 safetensors==0.5.2 sentencepiece==0.2.0 tokenizers==0.21.0 torch==2.6.0+cpu transformers==4.49.0 xformers==0.0.29.post3