@ViviNyehuusAndersen I understand the intention, but systems aren't elements in the model. If I'm not mistaken, only the worksets of the elements themselves when closed will unload the elements. Closing a system workset shouldn't hide the elements.
It is general it is good practice to keep all the HVAC elements on the same workset. Worksets can be subdivided in building sections when they get too large, but I wouldn't recommend changing system worksets (again, never played with system worksets before as it will probably create a lot of micro management work. So far I can't think of any benefits of managing system worksets).
Although closing worksets improves performance, a lot of the performance is also determined by view regeneration of which visible elements plays a big role i.e. hiding a linked model on a 3D view by turning off a workset in that view has the same performance benefits as closing the workset in the model.
For more information: Are you trying to disable all supply air ducting?