congestion, so 3G is faster than LTE
iPhoen 6s somehow locked onto 3G first (LTE connection timed out, or network kicked it to 3G, or other issues... eventually, if you have an idle connection, phone should upgrade itself to LTE)
you can maunally force nexus 6 to 3G and see 15mbps
or force iPHone 6s to connect to LTE, it connects to a different LTE band which could be more (or less) congested