Fortnite has just received a patch on Nintendo Switch that improves its performance in both docked and handheld mode by quite a margin. The update, which was rolled out yesterday, makes improvements to both the framerate and resolution and it will free up roughly 140MB on your console.
In handheld mode, the resolution has been improved from 1000x560 to 1170x660 (a 38% increase in pixels), and in docked mode, it has moved from 1390x780 to 1560x880. No specifics were given on frame rate improvements, but it's said to be more "consistent with fewer hitches, resulting in a smoother experience overall."
You can read more about the update here.