This is because your computer does not have to load these large files (usually high resolution textures) from a separate drive. However, storing your game and mods staging folder on your SSD will decrease sudden, massive lag spikes for your game. If your game is generally laggy with high resolution texture mods or ENBs, storing your game on your SSD will not fix these issues. A common misconception about running games from your SSD is that it will increase your game's performance (FPS, engine, etc.).