It is not "up to" 18%, it's supposed to be AT LEAST 18%. This has translated next to no practical change.
The 18% is also variable though. 'Up to' 18% infact.
PS5 has a whopping nearly 2X SSD i/o paper advantage, and we seen AC, a x-gen game on an old engine, gives a double figure 12% win, at best, or translates to a whopping 1.5s faster loading!
The difference is a XSX lead of ~18% in theorical peak for some GPU tasks and a lead of ~3% in CPU frequency.
But we also know PS5 has a ~22% lead in GPU frequency (so has a lead in other GPU tasks over XSX) and a lead in several important I/O areas, which means feeding faster GPU and CPU in a more optimized way, being able to free unneeded resources faster and replacing them via streaming with new ones. Which keeps it more time 'busy' with available resources to be used soon, so being able to reach a higher % of its lower theorical peak.
This means, in practical terms the performance difference is going to be tiny, in some games or areas with a console taking a small lead, and in other cases the other console taking a small lead.
Outside DF or NX Gamer comparision videos 99% of the players won't be able to spot the performance differences outside some small bugs or optimization issues that a specific game may have in certain areas (as could be tearing or tesselation in ACV).
Then we have the loading times differences, where PS5 will have a lead for native games but in most cases only of a few seconds, at least until devs start using Oodle Texture which will enlarge the difference. For BC games PS5 seems to shrink in a bigger % the PS4 Pro loading times, but since its games took way longer to load than the XB1X due to their difference in power, in many cases Series X loads them faster than PS5.
Then there will be the Quick Resume difference: in a console only works with a few recently played games and requires a big SSD chunk but resumes exactly where you left it and works with both BC and native games. The other instead isn't limited to the few recently played games and resume to the most recent savepoint instead but allows you to have quick access to multiple specific stages or game modes skipping menus and logo screens in all native games installed in the console but not with BC games.
So at the end, we get pretty much very similar results, sometimes better in one side, sometimes better in the other one. Or are different things where your preference may be for one or the another. There isn't a clear winner both on paper and with real world results.