Convert all Genesis' textures as JPG (even with compression at 100 if you want to retain quality like crazy). Replace in Blender your imported Genesis' textures with the converted JPGs. This will be much more manageable than the common huge PNGs and even worse TIFF textures that sometimes come with DAZ bundles.
Absolute nonsense, file format has no impact on vram!
Switch all subdiv modifiers to 0 for the viewport and turn on
Simplify in
Render Properties and set
Texture Limit to
1024 for the viewport (leave everything at max for
Render).
That being said, Blender's viewport is somewhat laggy even on high end machines, what are your specs?
Also, make sure to use the
Finalize Meshes option when setting up your characters, it can remove up to half the shapekey drivers of your figure and greatly improve performance.