The current implementation of DiffusionSolverFE may actually be faster than this legacy module. So before commiting to it please verify the performance of teh DiffusionSolverFE vs FastDiffusionSolverFE

FastDiffusionSolver2DFE module is a simplified version of the FlexibleDiffusionSolverFE steppable. It runs several times faster that flexible solver but lacks some of its features. Typical syntax is shown below:

<Steppable Type="FastDiffusionSolver2DFE">
   <DiffusionField Name="FGF">

In particular, for fast solver you cannot specify cells into which diffusion is prohibited. However, you may specify cell types where diffusant decay is prohibited

For explanation on how <ExtraTimesPerMCS> tag works works see section on FlexibleDiffusionSolverFE.