I doubt that hiding an entity will in any way speed things up when it is off the screen. Also, hiding an entity disables collision checking on it, which would affect gameplay.
Also, I wrote an octree entity hiding system a long time ago, and it was actually much faster to just use EntityAutoFade to hide thousands of trees than it was to try to hide them manually with hidentity. Of couse neither ran at a good framerate. It was just a test. This difference in speed though has likely been dramatically reduced because Mark changed the way Blitz handles hiding entities so that is near instantaneous. It was not when I did my earlier tests.
In summary, don't bother with frustrum culling. Just hide stuff based on distance, and occlusion if possible. (Ie, trees behind a hill can be hidden and that will speed things up.)
Also, I wrote an octree entity hiding system a long time ago, and it was actually much faster to just use EntityAutoFade to hide thousands of trees than it was to try to hide them manually with hidentity. Of couse neither ran at a good framerate. It was just a test. This difference in speed though has likely been dramatically reduced because Mark changed the way Blitz handles hiding entities so that is near instantaneous. It was not when I did my earlier tests.
In summary, don't bother with frustrum culling. Just hide stuff based on distance, and occlusion if possible. (Ie, trees behind a hill can be hidden and that will speed things up.)