Skyrim Performance Plus
Mod by VirusType2, aka 'Vergis'
Got some performance problems? Give these series of mods a try. They certainly can't hurt. Some users are reporting performance problems with certain effects like snow and rainfall. I had big problems with the falling aspen leaves in The Rift area of the game.
Skyrim was designed for consoles, which have good CPUs, but weak GPUs. It's generally the opposite for PC gamers, which have extremely powerful GPUs.
It's important to mention, the console versions run at only ~30 FPS, at lower resolutions, and indeed have framerate problems at times.
However, PC gamers play at higher resolutions, and use effects like Amibient Occlusion, Anistropic Filtering, Anti-Aliasing, and Post Processing Effect Filters. This really puts a burden on the PC. Especially because this game is already very demanding of a CPU. Even the real-time shadows are CPU intensive.
The special effects in this game, like snow and rainfall, are called particles, and are rendered by the already overtaxed CPU. Therefore, by reducing the textures in these particle effects, we can expect a variable degree of graphic performance improvement, depending on your system and graphic setup.