If their setup is working as they first mentioned it, they're using CPU generation so it's going to take longer to complete a step of an 512x768 image than an 8 year old GPU rendering at 1080p will.
Try adjusting this setting:
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My first guess is that it's just taking way too longer to render so Free Cities just gives up on receiving an image. Try adjusting that value to see if you'll get an image to show up. As for speeding up the generation, the best option is
not to use CPU generation. Unfortunately this is an area where the parallel processing capabilities of GPUs really show their strengths; even if you ran stable diffusion on a server with a $3,000 CPU, any GPU with 4 GB VRAM made within the last decade is almost certainly going to outperform your very expensive paperweight.
If that's not an option and you're going to stick with CPU generation, then see if using xformers helps. As you're not using a GPU, I don't think any of the arguments like --medvram will help as you're not using VRAM. You could change the generation settings as well, by reducing either the image size or using a different sampler than takes less math to run. On a similar note, you're also going want to decrease the regeneration frequency by
a lot by making that number in the settings a lot larger.
But at the end of the day, there's a really good reason why the people who are very into rendering images with stable diffusion either have or open their wallets for a GPU.