Valve released an update on September 25th that fixed volumetric fog rendering issues. The short message describes a graphical change, not new story content.
If you notice incorrect display of fog, start checking by installing the latest version of the game. The post doesn't claim that the patch improves frame rates on every headset or solves all performance issues. Compare the same area with the same settings. Mods and custom maps may have their own features that are not listed in this hotfix. For diagnostics, it is useful to write down the version, typeface and launch method. One visual effect change cannot be used to determine the minimum requirements of the entire game.
Sources and checks
September 25, 2026 Update · Steam