iPhone 15 Pro with 30 steps at 512x512 resolution (SD v1.5) should take around 35 seconds on iPhone 15 Pro with Draw Things. 1.5 mins are too slow. (I am the author of DT).
It could indeed be faster. The app does not currently use the neural engine (ANE) because it has a tendency to crash the app, so it uses only CPU and GPU. The app also does upscaling, which adds ~10 seconds.
I am going to put model related code we use in a public repo soon (it is very similar to https://github.com/liuliu/swift-diffusion but in NHWC format). ANE will be around 25s if it runs. DT's default only uses GPUs and 35s is on GPU (yes, like you said, upscaling would take extra 10s).
