Riften has notorious performance issues, and lots of mods try to fix this through various methods like removing clutter or smart usage of occlusion planes. These are all well and good but don't actually solve the underlying cause of the low frames, namely grass.

Beyond the walls of Riften where the player can't see, the game is rendering all of the grass, all of it. This would be pointless by itself, but when you've got grass overhauls installed, this is a killer for the city's performance.

This mod simply turns the grass off for the Riften worldspace, nothing more. I have little doubt this will fix the FPS problems for the vast majority of users, besides the ones who simply don't have the hardware to run the game (but even then, this should be a big help for such people).

The only visual difference the player will ever actually notice is that when you look through one of the lock-gates the tiny bit of LOD you can see won't have grass, but since that's what Whiterun does this should probably be considered what the vanilla behaviour was supposed to be.

Compatibility:

Should be compatible with near everything, water mods, weather mods, city overhauls, you name it. Just make sure it's placed right at the end of your load order. In the event the mod isn't working and you still have grass, crack open TES5Edit and see which mods are conflicting. The pertinent change you want to look for is under the riften worldspace section, and the 'no grass' flag.

Unfortunately, won't be compatible with Open Cities, because that mod removes the worldspaces, making this mod non-functional.