Some explanations what selected Basis Sets mean
“A basis set in theoretical and computational chemistry is a set of functions (called basis functions) which are combined in linear combinations (generally as part of a quantum chemical calculation) to create molecular orbitals. For convenience these functions are typically atomic orbitals centered on atoms, but can theoretically be any function; plane waves are frequently used in materials calculations.”
This overview sets the relation of what a basis set does and the calculation speed. Fastest for SCF  geometric optimization for C2H6 optimization with 2,4 sec was 3-21gsp, slowest with 11,4 h was d-aug-cc-pv5z (see Benchmark NWchem CH4 and C2H6).
- STO-3G – fast – slater type orbitals – 3 gaussian 
- 3-21G(*) – fast (2,4 sec) – Split valence: two sets of functions in the valence region provide a more accurate representation of orbitals. The basis is used for very large molecules for which 6-31G(d) is too expensive .
- 6-31g* – A significant improvement on 3-21G(*), 6-31G* adds polarization to all (non-hydrogen) atoms, and improves the modeling of core electrons. 6-31G* is often considered the best compromise of speed and accuracy, and is the most commonly used basis set. Available for elements H – Kr .
- lanl2dz_ecp lanl2dzdp_ecp
Links explaining Basis Sets
 Wikipedia: Slater-type orbital
 Encyclopedia: Self-Consistent Field