When building the mesh avoid cell alignments that generate very small cell faces. Small cell faces can increase run times and introduce instabilities.
HEC-RAS has feature that can filter these small faces, aligning cell faces so none of the cells are smaller than a specified (%) tolerance. Go to Map Tolerances under the RAS Mapper Options.
By changing the Minimum Face Length Tolerance to 20% (above) Mapper will adjust the cells from the red outlines (Length Tolerance = 0) to the black outline, removing the short cell faces and adjusting longer cell faces to compensate for those it removes.