LimeWire can't do much about the actual upload/download speed. The one with the fastest connection (with the lowest latency) get's the biggest share of the upstream.

First thing you do, if you want to investigate a problem, is checking the "Show Advanced Statistics" box in the statistics window.

If you want to know what's wrong with your uploads, disable ultrapeer capabilities and then wait for a little while until you have had some upload requests. Then go to the statistics and look at the number of push messages you received via TCP. If you receive any pushes (or at least more than 1 or 2 per hour) it means people have trouble establishing TCP connections to you.

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Morgens ess ich Cornflakes und abends ess ich Brot
Und wenn ich lang genug gelebt hab, dann sterb ich und bin tot

--Fischmob