I would recommend you remain with Dynamic Fragmentation on, there is no need to turn it off when you have a 4Mbs upload speed. When playing with time sensitive information like COD you would be better off having it enabled. While downloading a big file such as the one you were, maybe the router did not recognized that you were gaming and thats why it is always better to set up a game fuel, may I ask what console you were on? Anyways have a great day.
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Chris