Ensuring both reliable and low-latency communications over 4G or 5G Radio Access Network (RAN) is a key feature for services such as smart power grids and the metaverse. However, the lack of appropriate security mechanisms at the lower-layer protocols of the RAN–a heritage from 4G networks–opens up vulnerabilities that can be exploited to conduct stealthy Reduction-of-Quality attacks against the latency guarantees. This paper presents an empirical assessment of a proposed time-delay attack that leverages overshadowing to exploit the reliability mechanisms of the Radio Link Control (RLC) in Acknowledged Mode. By injecting falsified RLC Negative Acknowledgements, an attacker can maliciously trigger retransmissions at the victim User Equipment (UE), degrading the uplink latency of application flows. Extensive experimental evaluations on open-source and commercial off-the-shelf UEs demonstrate the attack’s effectiveness in increasing latency, network load, and buffer occupancy. The attack impact is quantified by varying the bitrate representing different applications and the number of injected negative acknowledgments controlling the attack intensity. This work studies a realistic threat against the latency quality of service in 4G/5G RANs and highlights the urgent need to revisit protocol security at the lower-RAN layers for 5G (and beyond) networks.