The workaround has been to only enable physical CPU cores by setting the affinity of the Subnautica process in Windows Task Manager. Subnautica and many other Unity based games are known to have really bad performance on systems with CPU's that have Hyperthreading or SMT. This can be adjusted using the HD Distance slider in the Graphics options menu instead of having to adjust it from the F3 menu.

The mod also reduces the distance at which high detail versions of objects are rendered by reducing the LOD Bias value from 10 to 1.3. The maximum values are Subnautica's default values. However, both the underwater and above water distances are configurable using sliders in the Graphics options menu so you can increase or decrease the distances to your preference.

The downside to reducing the render distance is that you need to get closer to things like the Aurora and islands before they appear. The mod also reduces the above water render distance from 1700m to 1000m to further improve performance while above the water. This can sometimes double the frame rate if you are looking in the direction of your base etc while more than 200m away.

You can only see about 200m away while underwater because of the water haze, so the mod will only render objects within this distance when the player is underwater. Subnautica also renders(draws) base components and other objects like the Cyclops and Seamoth up to 1700m away regardless of if the player is above the water or below the water. NOTE: This is a work in progress and there are issues with the rendering of the water surface while underwater if the underwater draw distance is set to less than 100 but the benefits were just too good to wait until all issues were fixed.

Subnautica game notes