Quantum effects play an important role in determining the double-gate (DG) MOSFETs characteristics. The non-equilibrium Green's function formalism (NEGF) in real-space (RS) representation provides a rigorous description of quantum transport in nanoscale devices. Unfortunately, the traditional NEGF framework has the disadvantage of being heavy in computations. Methods that reduce the computations exist in the literature like the recursive Green's Function (RGF) algorithm, the contact block reduction (CBR) method, and Gauss elimination (GE) method. Comparison of the simulation time of the traditional NEGF, the RGF algorithm, the CBR method, and the GE method was always theoretical and based on approximate estimates. In this work, we carry out a real comparison between the four methods by implementing them inside the same simulator, using them to simulate the same device dimensions and parameters on the same machine. It is demonstrated that the RGF algorithm or the GE method introduce about one order of magnitude reduction in simulation time below that traditional NEGF, whereas the CBR method yields the smallest simulation time with about two orders of magnitude reduction