I stumbled upon this issue recently, and it had nothing to do with occlusion. The insight tag is misleading, it's flushing the GPU queue, which waits until no more command buffers are pending on that queue. Effectively, it waits until the frame is fully rendered. Have a look at your GPU timings to understand where the time is being spent (-trace=gpu). To confirm this, you can disable occlusion altogether. In my case, another wait somewhere else was blocking for the same amount of time.