This is the fastest Flux [dev] method to this day, thanks to the lllyasviel.

Requirements:

1- Download the model and put it under models/checkpoints folder:  https://huggingface.co/lllyasviel/flux1-dev-bnb-nf4/blob/main/flux1-dev-bnb-nf4-v2.safetensors

2- Install BitsandBytes(BnB) with

pip install bitsandbytes

3- Update your Comfy UI

4- Install the custom node from Manager > Custom Nodes Manager> Install via Git URL > Paste the URL > Restart Comfy:  https://github.com/comfyanonymous/ComfyUI_bitsandbytes_NF4

Current limitations:

1- No schnell version is released by the original creator, lllasviel

2- Flux Lora's are not working with this workflow at this moment

Once again thanks for lllyasviel and comfyanonymous & comfyorg

Credits & Details:

https://github.com/lllyasviel/stable-diffusion-webui-forge/discussions/981

https://github.com/comfyanonymous/ComfyUI_bitsandbytes_NF4