The global adoption of 5G technology is rapidly accelerating and 5G traffic is growing exponentially. This increase in demand compels network operators to evaluate whether their current and upcoming 5G infrastructure can effectively accommodate the growing data traffic. A key component within the 5G network is the User Plane Function (UPF), which connects the end-devices to the data networks. Therefore, it is vital for both equipment manufacturers and service providers to analyze the performance of existing UPF implementations. This paper presents an initial approach to assess the Open5GS UPF performance by conducting stress testing in virtualized environment. We focus on finding the optimal UPF configuration by measuring the resource consumption and latency of the UPF under varying intensity of generated traffic, and providing a simple queueing model for the UPF. Numerical results show that a CPU load of 70-80% balances between latency and throughput while ensuring that 99% of the packets are forwarded within 150 ms.