The differences are exactly what you would expect, the phone that has been used for six years with a lower battery capacity performed *slightly* slower, but as Luke literally points out in the video, the only way you would notice is by putting the two phones side-by-side.
Which is obvious to anyone who has ever used technology, things get slower as time goes on.
And it certainly isn’t just iOS that’s the culprit, battery health is a culprit, larger application sizes, even completely external things like the fact that carriers have been slowing down their old LTE networks to make 5G look better.