have the same router.with both my 2.4 and 5.0 on..my 5.0 runs twice as fast as my 2.4 as it should..u should check ur settings..
i re-tested with 5 and it's looking better now. it was just testing from the xbox one "Detailed network statistics" test but i've seen various results so i think it was painting a deceiving picture of actual performance. my download speeds are cable broadband but nothing special, i.e. 30 Mbps / 5 Mbps.