Well, there are many mesh generation software out there. For most of my problems, I use Gmsh/pygmsh. To do scalability studies, i use the refine command to increase the size of the problem, rather than regenerating the whole mesh for each problem.
Depending on the application, usually built-in meshes doesn’t suffice to describe the geometries of interest. Therefore, in dolfin, you can load in meshes in parallel if they are in the XDMF-format. See for instance: Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio on how to convert msh files (from Gmsh) to xdmf.