Mod for Starfield that significantly improves performance, uses a combination of engine tweaks, compressed texture assets, and more.

Reportably gives pretty good 40fps results in non-demanding areas, and helps hit 30fps in demanding areas.

Unfortunately due to replacing many assets, the mod file size is huge (~7GB), but the author is preparing the download to be available through other file hosting services for people who don't have Nexus mod premium for the faster download speed.

Edit: SteamDeckHQ made an article covering the mod and it's improvements. Their findings were:

Compared to the stock version of the game, there have been some fantastic gains. With this version of the mod, I saw no drops below 30 FPS and some spots gaining about 5-8 FPS improvements overall. Without any TDP limits, it seems a stable 30 is finally possible with this mod, and I couldn't be happier.