A new performance mode has been added to Fortnite which compromises the visuals to improve the frame rate for those playing on low-end PCs. Epic detailed within a recent blog that it "offers significant performance gains by trading out visual quality to lower memory usage and lighten the load on CPU and GPU." The update, which is in alpha, is only available in Battle Royale and Creative mode, and it can be toggled between at any time within the settings menu.
An option has also been added to remove high-resolution content and this reduces the overall file size down to 18GB (providing that all components are removed). This option of course makes sense for those with less powerful machines, who are unable to utilise this level of visual polish.
You can read more about the update here.