In an operational mobile cellular network, measuring the mean mobile broadband performance at a certain place in a building within a short period of time (e.g., several minutes) is a challenging task as fading is position dependent and the overall network load changes throughout a day. In this work we derive a measurement methodology that allows for swift and repeatable measurements of the mean network performance by combining three techniques: Firstly, the instantaneous IP-throughput is derived within one second by using a novel packet pattern method on an Android cell-phone with external antennas. Secondly, the long-term trend in the IP performance, that is the slowly changing average network load, is removed. Thirdly, the scenario mean of the remaining small scale fading scenario with overlaid fast network fluctuations is obtained by statistical inference, namely, spatial systematic-sampling with an XY-positioning table. To assess the validity of our approach, we present the results of a seven day long live-network measurement campaign in Vienna, Austria. We show that, once the long term network trend is removed, a one minute long measurement is sufficient to determine the mean IP-throughput performance over the whole week with 10% accuracy at 90% level of confidence.