Since most loading times comparisons between Xbox One X HDD and Xbox Series X SSD, I took it a step further and tested some games with with different combinations.
Since some games also boot a bit faster the second time (probably when some data is still in the faster RAM), I rebooted the systems for the first run and then closed the game and started it again. If there were big differences in these two runs, I tested them again after another reboot to see if the results are reproducable. Sometimes the new results were a few seconds faster or slower (measuring tolerance), then I took the average time.
So here are my results:
The loading times of the two SSDs were almost identical on my Xbox Series X... so unoptimized Xbox games don't seem to take advantage of the higher bandwidth of the internal SSD. Only exception was Final Fantasy XV, which was a bit faster on the internal SSD.
The loading times on my PC SSDs also weren't much different.
And even the SSD loading times on my Xbox One X weren't much worse compared to the more powerful systems.
The next thing I noticed is that Red Dead Redemption 1 had the same SSD loading times on both Xbox generations. Since most 360 games are tiny (up to 10 GB), there ain't even a big benefit starting them from an SSD... an HDD is "good enough" for 360 games (and original Xbox games) on Xbox One X and on Xbox Series X/S. Huge parts of these games fit in the RAM, so the SSD advantage gets even smaller when starting the game a second time or restarting a level.
Eventually only series-optimized games will be installed on my internal Series X SSD. Unoptimized Xbox One games which I still play (f. e. Red Dead Redemption 2, Star Wars: Fallen Order) will be stored on the external SATA-SSD. And the rest of the games will be stored on my external HDD.