Ah nice comparison graphic, i hadnt seen many of these. I guess its nothing related to the fact the OS drive is still gen3 either then. Just how it is.
I did find on both drives the “bare bones” load time, and by bare bones in my case it was more like 250GB of official content (vs 122 or so really bare), was identical at 1min30sec too as a side note, though thats expected as well.