The fill rate of the video card is number of pixels it can render per second. Objects that are occluded (behind other objects) still consume fill rate. The total fill-consumption of the scene is the total screen real estate of all objects, including the occluded ones. Particles, in particular, can consume fill-rate like crazy, especially if the camera gets close to the particles.
In general, it is better for your graphics card to render the polygons front-to-back, because a depth test can be used to toss out the occluded fragments before they are written to the framebuffer.
When enabling the M_alpha or M_dual transparency modes, however, Panda forces the nodes with this transparency mode to be sorted back-to-front. This is necessary for alpha blending to work correctly. If you have many occluded polygons in view, for example thousands of blades of grass that are positioned behind each other, this may quickly consume your fill rate.
Do not enable transparency modes unless it is necessary, and when you do, consider using the M_binary mode, which does not require back-to-front sorting. However, if alpha blending is required and if large areas of the polygons are fully transparent, using M_dual may provide an improvement over M_alpha.
For a more in-depth explanation on the various transparency modes, see Transparency and Blending.