If you are on DSL bear in mind that it could be Asynchronous DSL -- where download is designed to be much faster than upload because most people spend most time downloading. Though the signal may look strong, the results suggest maybe not -- the indicators are not especially reliable:
Try to optimise your reception and avoid interference.
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.