Simulation time for 2D and 3D vastly different (with comparable DoF)

Hello,

I have a question regarding relative simulation time for 2D and 3D structure.

I am using COMSOL RF module to simulate an optical structure. When I try a 2D case with very fine mesh density and more than 4 million DoF, the simulation time only takes about 5 mins and very little RAM.

When I simulate a 3D structure with coarse mesh density with about 2 million DoF, the simulation takes about 30 mins with loads of RAM.

I am wondering how this is possible. Regardless of the dimension in 3D or 2D, the simulation time should, in my opinion, solely depend on the number of DoF. In my case, the 2D simulation has about twice the DoF than the 3D simulation, and yet the computation duration and memory consumption for the 3D is much more demanding. In both cases, I use MUMPS with exactly the same options, other than the 2D/3D difference.

I am wondering why this could happen.
Thanks.

Young