How can I turn off as many visual effects as possible?

Some mods or software that you could use to increase performance:

• CTD and Memory patch ENBoost

  Have you tired from CTDs when your game is heavily modded? Unable to install HD texture packs without CTDs or travel in the world very fast? This patch fixes the problem and also brings you higher performance even without memory fixes enabled. THIS IS NOT A GRAPHIC MODIFICATION, so feel free to use with vanilla game. THIS WILL NOT DEGRADE QUALITY IN ANY WAY TO INCREASE PERFORMANCE.

• Unofficial Skyrim Patch - a comprehensive bugfixing mod; also contains a "savegame debloatifier" feature that may help improve performance

• Skyrim Performance PLUS - installing it will give little (unnoticeable) graphics degradation, while certain CPU-intensive effects (falling rain, snow, leaves, etc.) are underway

• injectSMAA - like FXAA, but looks and performs better (disable FXAA if using this; if using ENBSeries, use the 'ProxyLibrary' .INI setting to also use injectSMAA with ENBSeries)

• Skyrim Project Optimization

  [uses] a feature called Occlusion Culling, where [it] manually puts occlusion planes in some spots of the interiors to improve performance. What these occlusion planes do is unrender parts of the cell/objects/NPCs that are not being seen by the player, in most cases behind a wall or above/below floors (occluded).

  If less things are being draw on-screen at the same time, it means your hardware is using less processing power to achieve the same thing, and it automatically converts in better performance.

These mods will increase performance, depending on a computer's specs, but will degrade graphics or turn off visual effects:

• Colorful Lights - No Shadows - More FPS

  If your computer spec lurks somewhere near or below the edge of the recommended minimum system requirements you have probably noticed that the option to disable shadows is not offered in the launcher, and consequently may have noticed that the low quality shadow settings look like garbage. If you scratched a little deeper you probably tried disabling shadows by changing the commonly recommended ini values and were disappointed to find everything had turned dark and colorless. The vanilla shadow options amount to menu choices between a garbled blocky flickering striped mess and a beautiful slide show, or editing the ini to induce a global dungeon vibe. This mod offers a alternative that I think strikes a nice balance between eye candy and system performance by removing most of the system-hog shadows. Everything will still be shaded to create the static, nice-looking shadows and colorful lights that utilize relatively little system resources. The shadows that are removed by this are the dynamically-rendered flickering shadows, commonly found near flames.

• Bethesda HD Textures Redux

  This is a cut down version of Bethesda HD texture pack for Skyrim. It aims at improving Skyrim visuals while, at the same time, allowing PC users with old systems to benefit from higher resolution textures.

• Interior and Dungeon Fog Remover
  Removes fog in dungeons and indoor locations; use this if getting low performance in these areas.

  Replaces most default "fog" meshes with blank ones.

  Several people have commented saying they are getting better performance in specific areas, so if you notice unusual performance drops in the presence of fog, this might help you.

• HiAlgoBoost FPS Performance Accelerator

  HiAlgoBoost dynamically modifies internal rendering resolution in order to increase frame rate, and utilizes various additional approaches to make the game more responsive and smoothly controllable. It does not affect game contents.

^{(Shamelessly copied from my answer in another question here.)}

There are three main areas where you can look to maximize your Skyrim performance: Settings, .ini, and Mods.

There is a rather nice and comprehensive tweak guide on the Geforce website that covers the first two areas. It goes into detail about which tweaks give you the best improvement vs lowest performance tradeoff. You can do that in reverse and find the settings that affect performance the most and turn them all down.

As for mods, there are a variety of mods designed to improve Skyrim performance on the Nexus Mods site. A simple search reveals these few(there are many more).

• Gigantic Skyrim FPS performance patch
• TESV Reduced Texture pack
• Skyrim Hyper Performance Boost for low end computers
• Skyrim Interior Fog Remover