5G is expected to expand the set of interactive applications enabled by the mobile network. Low end-to-end packet latency is a fundamental requirement for proper operation of those applications, but may be compromised in packet buffers shared with elastic applications that use greedy TCP sources for end-to-end transport. The accumulation of queuing delay by the elastic application degrades the latency for the interactive application, however light the throughput of the latter may be (as is the case with online gaming and over-the top voice). Buffer sharing is unavoidable in the RLC layer of the 3GPP RAN stack. To minimize its negative effect on interactive applications, we split the buffering between the RLC and PDCP layers. Then we equip the PDCP buffer with per-flow queues, and apply to the RLC buffer a new dynamic sizing mechanism that enforces the lowest queuing delay that is compatible with the existing configuration of the RLC connection. On a cellular link shared with a greedy TCP flow, our dynamic sizing solution can reduce the queuing delay of PING packets by up to two orders of magnitude compared to the default configuration with an RLC-only buffer of fixed size.