In mission-critical SaaS products such as payment systems, maintaining rapid release velocity while ensuring high reliability is essential. However, as these products grow more complex, the areas requiring regression testing expand, increasing testing costs and often compromising release speed. This paper proposes a methodology to achieve both rapid release velocity and high reliability by optimizing the regression test suite through defect severity analysis. Furthermore, a test architecture is proposed to balance unit, integration, and system testing. This approach reduces regression testing costs, shortens testing duration, and accelerates release cycles.