I just found this very interesting FAQ on dslreports regarding MTU
http://www.dslreports.com/faq/695
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For PPPoE, your MaxMTU should be no more than 1492, to allow space for the 8 byte PPPoE "wrapper", but again,
experiment to find the optimal value.. For PPPoE, the stakes are high: if you get MTU wrong, you may not just be sub-optimal, things like UPLOADING, or web pages, may stall, or not work at all!
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I wonder if this could explain the slow upload speeds some users have reported..