For nonlinear systems with input constraint, it is proved that the convergence speed of state under the action of a finite time stable controller(FTSC) is faster than that under the action of a non-FTSC at any point in state space. Sufficient conditions in six cases are summarized to guarantee the conclusion and it is proved that these conditions are attainable with appropriate parameter selection. Then, the definition of a complete control utilization controller(CCUC) is proposed, and it is proved that the convergence speed under the action of the FTSC is faster than that under the action of fast FTSC with input constraint. A series of conclusions concerning the parameter optimization are proposed to improve the control performance. Numerical examples, simulations, figures and tables are used to illustrate the proposed theory and make the conclusions more understandable. The proposed conclusions are fit for the improvement of nearly all kinds of FTSC methods, such as terminal sliding mode control, adding one power integrator control and finite time stable backstepping control.