There is no occlusion plane culling the Riften main square from the docks. More specifically, an occlusion marker is missing in the blacksmith building. For a regular mod build, this should not be a big issue, but if adding a lot of clutter to Riften (like JK's Skyrim or Dawn of Skyrim), the FPS drop is noticable. For my own build, I solved the issue by adding a few occlusion planes or cubes to said building and inside the wall. I also added a few planes to other areas in Riften where I thought it was helpful (the biggest gain was seen from a few in connection to the orphanage). I have attached the file I used for this, so feel free to incorporate its additions into the mod. It also includes one occlusion plane near Markarth, which helps with FPS issues just outside its walls.