Performance testing of an e-commerce application should be conducted once functional testing of major features is completed. However, planning for performance testing should commence from the initial software development phase. The performance test plan should encompass details about server specifications and architecture, including the expected user load on normal days and during sales occasions.
Performance testing should be carried out in a live-like environment as it helps to simulate actual behavior. During performance testing, make sure to involve the network team to monitor the load.