I’ve used that script to train AlexNet before - I found that it’s such a simple model that it’s very easy to be limited by CPUs and/or disk access, and the number of GPUs is often irrelevant.
You can try increasing the number of threads which fetch the data (e.g. -j16), or you could try using a larger model if you want to check whether multi-GPU speedups are possible (e.g. --arch=resnet50).