If you take advantage of the way Skyrim Special Edition handles heavy mod loads to tweak the heck out of it—and especially if you use an ENB to overhaul the graphics—you've probably taken a hit to your framerate. (I've been using Ominous ENB to give things a gloomy, somber look, but it costs up to 20fps in some areas.) If that's you, exterior FPS boost is here to help.
It works thanks to occlusion culling, which prevents geometry you can't see from being rendered. While Skyrim already has occlusion culling, eFPS adds significantly more. Have a look at the changelog in the mod's description to see just how many cells have had occlusion panes added to them. It's a lot.
The existing Project Optimization mod does the same for interiors, and the two mods work together. However, eFPS is incompatible with mods that rearrange structures, like Open Cities and JK's Skyrim. The description also recommends starting a new game, since "We still haven't completely figured out how eFPS behaves with existing saves." I didn't notice any ill effects on my current save, however, and picked up about 5fps in Whiterun even with Ominous ENB making it all dark and smoky.
That's not a huge improvement, but your mileage may vary. As the mod's description notes, "the increase in performance is inversely proportional to the power of your computer. If you have a very very very powerful PC, you may only see a small improvement in FPS. On the other hand, the idea of this mod is to improve performance especially for those who do not have an ultra-powerful PC."
To make sure your framerate's hurting in the first place, here are some of the best Skyrim Special Edition mods.