A fixed grid with a fine mesh may provide accurate numerical solutions for certain problems. High computational costs, however, may be prohibitive for complex multi-dimensional simulations of large-scale problems. Speed is largely related to the number of nodes in the model. If the user proceeds with adding 5x more nodal density to the model then the solution time would be expected to grow by at least 5x. Automatic mesh generation and refinement greatly enhances model stability, accuracy, and convergence by automatically refining the mesh only where needed.
More information on AMR may be found in the following articles: