Original Post
i did 2 things
1. disable depth testing for drawing the skybox(i use a cubic skybox, but i think you need to enable backface culling otherwise it shouldnt work
2. don t clear your colors only depth buffer
that works fine in my engine i got a performance boost of 15 fps on high poly landscapes
and 30-35 on low poly
the difference of the performance boost has to do with the amount of polygons that are rendered the more polygons the less glClear calls -> less performance boost