According to research conducted by RootMetrics, AT&T has the fastest pure LTE speed of any of the major carriers, clocking in average download and upload speeds of 18.6Mbps and 9.0Mbps. Number two Verizon delivered 14.3Mbps and 8.5Mbps, respectively, with Sprint a distant third at 10.3Mbps and 4.4Mbps. T-Mobile doesn’t offer 4G,...