whether it reads and processes images over and over …

yes, there is no way around it unless you can fit all data in your memory

could it make a speed issue …

yes, it will be somewhat slower depending on how fast your hard drive is

Do you know the way?

Not really, most of us probably deal with datasets that will never fit in memory. You could add optional parameter to your dataset that instructs it to load everything in constructor for situations when you’re sure your RAM can handle all of it (probably measure the time difference beforehand to make sure you’re not wasting time on something that will bring only marginal improvement).