What I mean is, if you download a file that is say, 20MB, does it slow down after maybe 10 or 15mb? If so, there's a file size throttle. Comcast used to have it where any file under 20mb downloaded at full speed, but after that the download speed would be cut.
If the YouTube and Vevo videos are above a certain download size, it might be T-Mo's throttling your speed. In either case, contact them about it before blaming your phone since they do have throttling policies in general.
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If it ain't broke, use a sledge hammer.