For any viewport you can disable lighting like this:
- Select viewport
- Go to the inspector window
- Find the "rendering" fold-out group
- Find "debug draw" at the bottom of the new group
- Click "disabled" to open the menu
- Select "unshaded" in the menu
Your viewport should no longer use lighting.
But, you can use other stuff for viewport optimizations too if it's still laggy. Probably most useful will be making the viewport's size smaller, or putting the Viewport in a ViewportContainer and changing "stretch shrink" to a number like 4.