Several novel userspace network stacks have been proposed in recent research to overcome the limitations of the Linux network stack in providing high-performance I/O for Virtual Network Functions (VNFs). In this paper, we evaluate the performance of several state-of-the-art network stacks in the context of the VNFs of the 5G mobile packet core. The VNFs in the 5G core are several times more compute-intensive than the VNFs used to benchmark network stacks in prior work, given the need to perform user authentication and other such cryptographic operations. Our evaluation shows that while modern stacks outperform the Linux kernel stack over I/O intensive VNFs (as observed in prior work), the performance gap is not as wide in the case of CPU-intensive VNFs of the 5G core. We also find that the packet core VNFs can obtain up to 67% higher performance if the network stack could partition traffic to CPU cores at the granularity at which VNFs maintain state (mobile subscriber in this case), enabling a lockfree architecture within the VNF. The insights from our work can help us design a network stack that is better suited for compute-intensive VNFs such as those in the 5G core.