Unfortunately all those mass consumption dataset loaders and feeders are far from optimal, everyone serious about this ends up writing their own performant code (or compensate by getting expensive SSDs and CPUs) but don’t publish it because it’s embarrassingly written.
People often say you need to use SSDs and whatnot, but I use a couple of ancient HDDs and it’s enough to feed a duo 1080 setup. It’s just troublesome to write a properly multi threaded application for this, you don’t even need to do it in C if you have a good CPU.