6.1 On Performance
6.1.1 TEXTURE SIZE
Depending on their size and resolution, simple texture replacer can impact performance, but will not necessarily do so. The vanilla textures are 7.19GB in total when packed in BSAs and 9.84GB as loose files. Most textures are 2k or 4k textures, though many of the latter are upscaled which means they were originally smaller and will not look as good as textures crafted for 4k.
Texture resolutions are always a power of two:
1024 (210) - textures with a 1024x1024 resolution are commonly abbreviated as 1k
2048 (211) - textures with a 2048x2048 resolution are commonly abbreviated as 2k
4096 (212) - textures with a 4096x4096 resolution are commonly abbreviated as 4k
8192 (213) - textures with a 8192x8192 resolution are commonly abbreviated as 8k
This also means that one texture in 4k resolution (e.g 160MB) has four times the size of the same texture in 2k resolution (e.g. 40MB). While the visual improvements of 4k over 2k may be subtle ingame (unless you zoom in close for a screenshot) it makes a huge difference when it comes to how many megabytes of textures your graphic memory has to load up. When your harddrive struggles to access the textures in time and your GPU can barely load them all into its memory, you will run into performance issues.
These issues are most commonly stutters, meaning the game is unable to render the world or objects in time. Stutters when standing still and looking around or switching to a weapon with a high-res (re)texture are most likely due to your graphic card's VRAM being overloaded.
6.1.2 TEXTURES - CONCLUSION
Now all this does not mean that you have to limit yourself a lot or should never install 4k textures. Just be reasonable when choosing a resolution. So long as you have at least 3GB of VRAM, you can absolutely use 4k textures for things like buildings, large creatures, landscape, and so on. For any weapon, armor, furniture or interior textures, a 2k resolution might be appropriate. And for anything smaller like clutter, gems, coins et cetera, you'll want to use textures with a 1k resolution or smaller.
For people with extremely low-end PCs (2 GB VRAM or less), it might be beneficial to install smaller textures (even smaller than vanilla) to improve performance.
Meshes can also have a performance impact depending on their complexity. You may loose a few FPS when running many 'high-poly' (very detailed) meshes at once. You can read more about Polygon meshes here because I really don't have a clue.
Occasionally meshes can actually crash the game (repeatable CTDs) due to them having a structural error or not being optimized for SSE. These issues are typically (comparatively) easy to troubleshoot.
6.2 How do I choose which resolution to download?
People's preferences vary a lot - some like their game with stable 60FPS at the expense of prettier textures, while others are fine with a lower frame rate but many large textures. You will have to find the sweet spot for yourself and figure out which combination of mods results in a visually pleasing Skyrim that performs as well as you need it.
For example, most people with decent rigs and a 1080p / 1440p monitor should probably go for a balanced setup: That means 2k textures for armors, weapons, furniture, carpets and so on. Choose 4k textures for large objects, such as mountains, landscape, dragons, mammoths, houses, rocks. Since you need less VRAM in interiors (because less objects and no LOD) you can go for 4k caves / mines textures as well. For small items like gems, food, arrows and other clutter objects I would recommend using mostly 1k textures.
Check out my general guide line below.
From this point on only the Balanced options are included in the instructions.
You are however free to download a lower or higher resolution version of any mod.