At any time, one stage of the pipeline is the bottleneck. Reducing the time spent in the bottleneck is the only way to improve performance. Speeding up operations in other parts of the pipeline has no effect. Conversely, doing work that further narrows the bottleneck, or that creates a new bottleneck somewhere else, is the only thing that further degrades performance. The workload can be increased at other parts of the pipeline without degrading performance, as long as that part does not become a new bottleneck. In this way, an application can sometimes be altered to draw a higher-quality image with no performance degradation.
Different programs stress different parts of the pipeline, so it's important to understand which elements in the graphics pipeline are the bottlenecks for your program.