Some of these features are covered here:
https://supertuxkart.net/Antarctica:_OverviewBut here's a more complete list (performance costs are anecdotal, and vary depending on factors such as hardware used, GL/GLES build. etc.):
Advanced pipeline: Lighting, shininess, etc. Disabling this causes STK to use a very basic renderer that basically just shows the textures as-is, though this can significantly improve performance on very low-powered hardware.
Shadows: Pretty self-explanatory here, enabling this renders dynamic shadows. Medium-high performance cost.
Bloom: Creates a bloom/lens flare effect around bright light sources. High performance cost.
Light shaft: Renders the sun and the related lens flare effect when looking at it. Low performance cost.
Ambient Occlusion: Makes crevices/etc. dark. High performance cost.
Depth of field: Creates a Bokeh (blur) effect on distant objects. Medium-high performance cost.
Glow: Provides an outline effect on collectibles (Gift boxes, bananas, Nitro cans, gum, etc.), often making them easier to spot. Low performance cost.
Anti-aliasing: Avoids aliased lines when rendering. Low-medium performance cost.
Motion Blur (Unintuitively) Enables a radial blur effect when you activate a speed boost. Low performance cost.
Image-based Lighting: Causes metallic objects to reflect a texture rather than just a solid color. On desktop (a.k.a. non-GLES) builds, also enables Screen-space Reflection. Low (GLES) / medium (GL) performance cost.
Light scattering: The air around a light will glow. Performance cost largely dependent on how many lights a track has.
Animated characters: Whether karts/etc. have their animations enabled. Very low performance cost.
Texture compression: Compresses textures on the GPU. Enabling can improve performance (especially on iGPUs), though at the cost of slightly reduced texture quality.
Particle effects: Enables or disables various particles. Very low performance cost.
Rendered image quality: Unintuitively, this many controls the quality of textures (anisotropic filtering, maximum texture size, mipmapping quality, etc.)
Geometry detail: Enables or disables rendering of various extraneous objects. To ensure no issues when playing online, this option only takes effect when playing offline. Low performance cost.
Contributor to/fan of STK (Upstreamed Cartoon theme, numerous random big fixes/tweaks)