Programs that cause bottlenecks at the rasterization (per-pixel) stage in the pipeline are fillrate limited. To test for bottlenecks in rasterization operations, shrink objects or make the window smaller to reduce the number of active pixels. This technique won't work if your program alters its behavior based on the sizes of objects or the size of the window. You can also reduce the work done per pixel by turning off per-pixel operations such as depth-buffering, texturing, or alpha blending. If any of these experiments speed up the program, it has a fill-rate bottleneck. For more information, see ``Tuning the Raster Subsystem''.
Many programs draw a variety of things, each of which stresses different parts of the system. Decompose such a program into pieces and time each piece. You can then focus on tuning the slowest pieces.