How to speed up passing of images to a GPU
Try to use batch version for training. You have to implement a function to manage your data which exist in the memory. For each batch, you can load the data into your memory as much as possible, high RAM size can help. Then you can pass a batch to your network, the size of graphical memory matters here, using the current data which is in memory now. By doing that you reduce the number of disk calls. When it happens to load data from disk, you fill the memory as much as possible, the disk speed matters here; consequently, high speed SSDs are valuable for facilitating the reading operation.