Abstract—5G mmWave technology promises capacities 10 to 100 times that of 4G. However, mmWave links are very sensitive to having direct line of sight between sender and receiver, with dramatically fluctuating capacities due to transient blocking and shadow fading, which can substantially degrade TCP performance. TCP-splitting proxies (SPEPs) are often used to improve TCP performance over wireless links. We investigate current operator SPEPs (used in 4G LTE networks) under mmWave-like dynamics using the MONROE testbed. We introduce a mmWave emulation model for an urban canyon scenario, and use it to evaluate how current operational SPEPs impact the performance. Our results from experiments including four operators and three countries, indicate that SPEPs will be even more important for 5G mmWave than they are in LTE. Current 4G SPEPs provide significant, though not optimal, benefits under mmWave-like conditions, allowing them to be utilized as the network transitions to 5G technology.