I would guess it's one of three things:
- You are using Windows instead of Linux for ROCm (I don't know how much this affects performance, as I am only on Linux)
- You are generating the batch all at the same time, instead of just doing multiple generations. This can lead to out-of-memory issues, as it is a larger image being generated.
- You are not fully using the GPU. Does task manager say 100% utilization?