Seems like most of the performance guides on here just say to lower the DLSS mode, but this game actually offers a lot more configuration than that.

Please note:
DO NOT TOUCH the normal Portal graphics settings, they do nothing and will cause instability, most people reporting crashes probably did this. Instead, press Alt + X to open Nvidia's new overlay settings menu.

Disclaimer:
I did not figure any of these settings out myself nor do I fully understand what they do, credit goes to user Pixels in the comments section of Digital Foundry's video on Portal RTX. According to him, many of these settings use 4000 series exclusive features which hurt performance on older cards with no visual benefit as well as settings that are unnecessarily high, I wanted to post these here so more people could be aware of them.

*Update: After the 1.05 patch, several of these settings are disabled by default on lower graphics presets, making this guide less necessary.