Entities are culled by the BSP process.

But there's a bug in the 6.30 version that causes a heavy slowdown when the entities are ent_created, or have an action. This is caused by a physics bug (even it the entities don't use physics), and will be fixed in the next release.

But if you experience the problem also with older versions, like 6.22, there must be a different reason. The next version will have a more complete statistics panel that shows by which part of the frame cycle the slowdown happens.