As of fall 2024, there are nearly 100 Meta Quest 2 games we know of with specific Quest 3 and 3S patches for improved performance and graphics, and the total number could be higher. Plus, there are dozens of Quest 3S games with mixed-reality modes that look much worse with Quest 2 passthrough.
With the Meta Quest 3S, your games library will have faster loading times by default. However, some developers patch their games to fully exploit the extra power for higher refresh rates, new textures, better lighting and shadows, and other visual upgrades that the Quest 2 couldn't handle.
Meta confirmed in its Connect 2024 blog post that any "games ... that received Quest 3-specific performance updates will also work day one on Quest 3S, which means you’ll benefit from the same upgraded visuals and faster load times."
We also spoke with Meta VP Mark Rabkin at Meta Connect 2024 about this, and he confirmed that there is "100% compatibility between the 3S and the 3" in terms of game performance.