Missing an achievement after doing the work is a miserable way to finish a challenge. Valve has listed a fix for that exact problem with Gnome Alone in Half-Life: Alyx. It's one of the changes in the October 26 patch, alongside fixes for rendering crashes in Workshop content.
The notes don't explain whether affected saves need another action to trigger the achievement. If yours is still missing after the update, keep that distinction in mind: a listed fix doesn't tell us what happens to every existing save. Valve's announcement is the source here, and we'll avoid inventing a recovery procedure.
A new switch under Advanced Performance
The other addition gives PC owners something to try before changing all their graphics settings. Foveated rendering is now available on supported hardware. Valve names NVIDIA GTX 16-series and RTX 20-series or newer cards for DirectX. AMD needs -vulkan in the game's launch options.
Open Performance and use the gear icon to reach Advanced Performance. The configuration guide linked by Valve says the switch is off by default. It also says the feature can work with any VR headset when the graphics card supports it. Eye tracking helps, provided the driver supplies the tracking information to the game.
Keep the comparison simple
Try a familiar save with your usual settings first. Change the new switch, repeat the same section and see whether you prefer the result. Keep an eye on the image while moving your head, too. Getting a nicer performance number isn't much comfort if an effect bothers you throughout a session.
That's our suggested first check, rather than a benchmark result. Valve says performance can improve without attaching a single guaranteed gain to the feature. The patch also includes a reprojection-related change for Alyx running natively on Steam Frame.