DarkRadiant Tips and Tricks
From The DarkMod Wiki
Rendering Performance
For large maps the rendering performance of DarkRadiant can be poor, depending on your system. Use this to give DarkRadiant a small powerup:
Got to Edit -> Preferences and then select Orthoview
and make sure the Update Views on Camera Move is unchecked.
Disabling this option will stop camera movement from triggering an update of the orthoviews, which is expensive.