Could you describe your use case a bit more and explain why you want to store all ImageNet images as tensors?
Note that the size would most likely increase (depending on the original image format) while the actual loading time would depend on the bandwidth vs. decoding performance of your system.
E.g. a JPEG image in the shape [800, 800, 3] uses ~107kB while loading and storing the same data as a tensor uses ~1.9MB.