How to speed up data loading from disc?

So i need 5 sizes of every picture.
I preprocess and save everything with numpy, but loading files from disc (probably) take most time (much more then model/tensor processing).

My dataloader looks like this:

class GanDataset(Dataset):
	def __init__(self, samples):
		self.samples = samples

	def __getitem__(self, index):
		sample = self.samples[index]
		return sample
	def __len__(self):
		return len(self.samples)

class GanCollate():
	def __init__(self, sizes):
		self.sizes = sizes
	def __call__(self, batch):
		output = []            
		for size in self.sizes:
			output.append(torch.stack([torch.from_numpy(np.load(os.path.join(str(size), i) + '.npy')) for i in batch]))
		return output

dataloader = DataLoader(GanDataset(pics), batch_size=batch_size,shuffle=True,
						num_workers=workers, pin_memory=False, drop_last=True, collate_fn=GanCollate(sizes))

Full code