Modern Vintage Gamer took a look at Switch 2 backwards compatibility:
tl;dr-
If a game is memory or GPU bound, performance is generally greatly improved. While the game won't exceed it's set FPS or designed resolution, this can GREATLY improve games that suffered inconsistent frame rates. Games like Bayonetta 3 and Borderlands 3 run 60fps, while Doom 16 and Batman Arkham Knight hit stable 30 fps.
Games CPU bound (Dragon Quest Builders 2 featured, but simulation games like Cities Skylines or Civilization 6 probably also) are not going to see THAT much an uplift since the actual CPU clocks are fairly similar between the systems. That said, the improvements in other areas DO seem to allow for better overall performance in most situations.
And since Micro SD Express is a newer, faster, storage medium, load times are generally improved for older titles.