Loading happens when games need to fill memory. Ideally, memory size increases come along with storage speed increases. When you moved from PSX to PS2, for example, the RAM to fill went up by something like 12x but the drive speed went up by about 16x, so load times went down slightly.
Disc drives barely got faster between 360/PS3 and XBONE/PS4, but memory got roughly 12x larger, so loading times would have dreadfully increased if games loaded from discs, and were only mildly salved by forced installs and loads from spinning platter hard drives.
Next gen is moving to solid-state drives (I believe that’s known since that PS5 leak), which is a massive increase in read speed. I don’t know what the memory budget will look like but given that desktops have barely moved it won’t possibly be more than 3x faster.
The bottleneck for the foreseeable future, then, will be CPU – time to decompress the assets. On a modern desktop you’ll still see around 5s load times no matter the disk speed.