Step 1: Start the game, press ALT + X, disable upscaling or set it to "Fullscreen" and set the Quality to medium. Save the settings
Step 2: close the game
Step 3: open AMD Drivers and search for "Portal with RTX"
Step 4: enable HYPR-RX for this game - this will enable driver level upscaling
Step 5: Start the game
Step 6: lower the resolution of the game in options - the driver takes the upscaling task wich results in a superiour quality to perfomance ratio on AMD GPUs
Step 7: load into the game
Step 8: play around with the resolution until you get a good framerate, keep the game on fullscreen and let the driver handle the rest.
Step 9: Press ALT + R, search for "Portal with RTX" - under "Radeon Super Resolution" play around with the sharpening-effect until the image quality fits your taste. You can press Alt + R to close and open the overlay

Note: you may have to press ALT + X and disable Nvidia's upscaler after every launch of Portal with RTX

Minimum recommended GPU for 1440p: RX 7800 XT