Really slow attacks with demo code

I'm trying to run the demo in attacks.ipynb, but it is really slow, taking over 30 minutes to run the [PointwiseAttack](https://github.com/bethgelab/AnalysisBySynthesis/blob/458ee76af68778fd9a0ea7c4b411fa4c7eeb8682/scripts/attacks.py#L73).

I'm using GPU and running the [classification](https://github.com/bethgelab/AnalysisBySynthesis/blob/458ee76af68778fd9a0ea7c4b411fa4c7eeb8682/scripts/attacks.py#L66) was quite fast, but I'm wondering if I may be doing something wrong, or is it normally like this?

Thanks

Start with attacks.ipynb and the PointwiseAttack entry point in scripts/attacks.py around line 73, then compare it with the classification path around line 66. Reproduce the demo with the reported GPU setup and inspect the runtime difference. Done means determining whether the PointwiseAttack duration is expected or identifying what setup or execution detail causes the slowdown.

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