To understand the impact of texture bandwidth on the rendering time (large items such as the ground or a wall are good examples), do the following:
Select Full Tree from the drop-down list in the Scene Overview pane.
Select all ergs. Ergs will be highlighted in the Visualization pane.
Go to the Texture tab and right-click on a big texture and chose "Select all Ergs that use this texture" from the context menu.
Go to the Render Target Viewer pane. From the Selected ergs drop-down list select Normal.
In the Texture tab, move the Clamp to Mip slider to a large number.
The largest number on the slider corresponds to the smallest mip level.
When the scene re-draws, you should see the detail disappear, and the geometry will be a flat color at the smallest mip level.
If reducing the mip level for the selected ergs significantly reduces the rendering time, then texture bandwidth may be a bottleneck, and if the change in visual quality is acceptable, you can gain speed by using a smaller texture.
If there is little or no visual difference in the scene when using a smaller mip level, then you are wasting texture bandwidth for this scene, and you can regain some texture memory by using a smaller texture. This usually happens when geometry is rendered smaller than expected. The hardware (in standard filtering modes) always chooses the best match for the screen pixels being displayed. So, the top-level mip will not be used if the geometry being rendered is significantly smaller than the resolution of the texture.