To reduce calculation time you can specify a smaller calculation domain to reduce the number of computational cells. By analyzing a calculation with a coarse grid (large cell size), e.g. with a cell size of 5 or 10 m, you get an idea where the flow path is situated and you can limit the calculation domain to the area of interest.
Switch to 2D mode and choose Input → Calculation Domain... → Draw New Domain. Now you can draw a polygon containing the area of interest similar to drawing a new polygon area (see exercise "Create release area" on page 34). We strongly recommend using smaller calculation domains especially if you calculate with small cell sizes (e.g. < 5m).