Xbox App 2112.1000.22.0 - noticeable improvement in overall speed

Xbox remove this featureto improve the Xbox's overall speed and resources and there is currently no sign of it coming back

I agree, I have played destiny religiously everyday since it dropped and I have definitely noticed an overall improvement in my fps skills across the board. Everything from awareness to reaction times has dramatically improved. I definitely think playing
a lot of bullet hell games has helped with reaction speed as well.

If the 15MBps is your overall line speed, then that would explain the lag as that is quite a low speed for cloud gaming so buying a console would improve game performance

To quote Mike Ybarra it was removed to "improve multitasking, reduce memory use, improve overall speed, and free up resources going forward for bigger things."

You can read more here: https://www.gamespot.com/articles/microsoft-is-removing-xbox-one-snap-mode-and-heres/1100-6447261/

Xbox replaced Snap to improve multitasking, reduce memory use, improve overall speed, and free up resources going forward for bigger things. It’s not clear what those “bigger things” are just yet, but rumors have suggested Microsoft might opt for some type
of picture-in-picture mode to let universal apps run alongside games.

Also to answer your question, if you purchased your Xbox at Walmart and kept the receipt, you have a week to return your console for a full refund.