Regarding your questions, yes, DEM also benefits a lot from GPU acceleration. So you can compare it to a CPU based code, but obviously there's an order of magnitude you can gain via GPU.
Usually you are not interested in the fine fields anyways. Think of some fine powder in a big process, where there are trillions of real particles inside. You can't and don't want to simulate that. Mostly you are interested in these course quantities anyways and getting statistical data, so for that there's no need for the fine resolution.
Regarding the numerical model that can compute these things in a more efficient way, they don't always exist. When you move to large numbers of particles you can sometimes go to continuum models, but they might not always behave as the real thing, as it's really difficult to find governing equations for such materials.
Usually you are not interested in the fine fields anyways. Think of some fine powder in a big process, where there are trillions of real particles inside. You can't and don't want to simulate that. Mostly you are interested in these course quantities anyways and getting statistical data, so for that there's no need for the fine resolution.
Regarding the numerical model that can compute these things in a more efficient way, they don't always exist. When you move to large numbers of particles you can sometimes go to continuum models, but they might not always behave as the real thing, as it's really difficult to find governing equations for such materials.