The virtualization of the local area network is predicated on the **IEEE 802.1Q** standard. By inserting a 4-byte shim between the L2 MAC header and the payload, we gain the ability to multiplex thousands of isolated logical networks over a single physical wire. However, at the limits of line-rate 400Gbps networking, these 4-to-8 bytes represent a recurring goodput tax that must be accounted for in performance models. This article provides a clinical engineering model for calculating the **Layer 2 Efficiency Penalty** of tagging and explores the mechanical impacts on MTU and hardware ASICs.