Modern programmers routinely use third-party code, and infrastructure operators deploy software they did not write. This would not be possible without semantic interfaces---documentation, header files, specifications---that succinctly describe what that third-party code does. We propose performance interfaces as a way to describe a system's performance, akin to how a semantic interface describes its functionality. We concretize this idea in the domain of network functions (NFs) and present a tool (PIX) that automatically extracts performance interfaces from NF implementations. We evaluate PIX on 12 NFs, including several used in production. The resulting performance interfaces are accurate yet orders of magnitude simpler than the code itself and take minutes to extract. We show how developers and operators can use performance interfaces to identify performance regressions, diagnose and fix performance bugs and identify the latency impact of NIC offloads.