There are various optimization methods in the category of "culling": ignoring faces that are not visible to the camera:
Backface culling - hide triangles with normals facing away the camera
Occlusion culling - hide triangles obstructed by other objects
Frustum culling - hide triangles that are out of the camera "cone"
Frustum and ...
You can use the command line for this.
To render only the transition frames :
blender -b c:/mypath/myfile_without_mblur.blend -f 101,102,501,502
To render in-between frames :
blender -b c:/mypath/myfile_with_mblur.blend -f 1..100,103..500
Command line rendering documentation
Here is a try, create a low-poly object, give it a material with Metalic value at 1 and Glossy at 0, give your world an HDRI so that your object has something to reflect:
As Markus says, your object is actually completely flat, so what you could do is first create more topology (perhaps with a CtrlT to triangulate) then select some vertices (for example ...
I don't think the regular Shade Smooth command works on geometry created by Geometry Nodes unfortunately.
Currently, you will have to connect your ico sphere node to an Attribute Fill node. Set the node's second option to Boolean and set the Attribute to shade_smooth, then you'll be able to click the box next to Value to toggle between Smooth and Flat ...
If backface culling is off (which it is by default), yes. Backface culling was created specifically to help with this. However, with transparent or reflective materials, it can cause both undesired or intentionally unrealistic and thus cool effects.
I guess pixel differences comes from several facts
animation (movement changes)
Cycles with quite low samples and most of that
Dark place with low samples generates more noise (more than in other places like completely black or lighted areas). Too much noisy area that runs trough denoiser will produce more visible distortion in reconstruction....
All you have to do is turn off SSS (subsurface scattering), or if you want to keep it, lower it down and wait patiently for it to render. Also, see the render samples to something like 128 and enable Intel Denoising tool in the compositor with all the settings and it should be fine.
You need to enable the Backdrop option, shown in the screenshot with the right-marker. And also link an output, Viewer node to your input, Render Layer node, shown with the left-marker in the screenshot.
This should display the rendered image in the background of your compositing editor which allows quick tweaking of the image without needing to switch ...