I have about 100 .npy files that contain CSR matrix data. Data have been saved in multiple files because np.save doesn't allow more than 4gb file. Now to train a classifier, I need to load the data from all .npy files to RAM. I am using vstack to create a combined CSR matrix, but it is slow. Can someone please let me know what is effective way to use vstack or any other method to create a combined CSR matrix using the data from all 100 .npy files?