what is the difference between layer off and layer freeze?
To freeze a layer, click the snowflake icon to the right of the yellow light bulb. When you freeze a layer, the visible effect is the same as turning a layer off. The difference, however, is that when you freeze a layer, AutoCAD releases it from memory. If you refresh a drawing after freezing a layer, the layer won't reappear. If you freeze a layer instead of turning it off, you'll see a boost in performance because the program no longer has to keep track of it.