While trying to update a few packages, (not only) I noticed that it's very slow to download from this mirror, although it's said to be geographically located near me:

$ traceroute us.archive.ubuntu.com
[...]
11  ae-94-94.ebr4.SanJose1.Level3.net (4.69.134.253)
[...]
17  ae-82-82.ebr2.Washington1.Level3.net (4.69.134.153)
18  ae-44-44.ebr2.Frankfurt1.Level3.net (4.69.137.61)
[...]
25  ae-42-42.ebr2.London2.Level3.net (4.69.141.194)
26  ae-26-56.car2.London2.Level3.net (4.68.117.176)
27  gi1-0-1.oxygen.canonical.com (195.50.121.2)
28  drescher.canonical.com (91.189.88.40)

So, I guess I have to pick another mirror then.

Update: A nice fellow on #ubuntu-mirrors was kind enough to answer this one for me: apparently no single US mirror would like to act as us.archive.ubuntu.com because of the high (network) load involved, hence Canonical is providing this "mirror", although it's not really a mirror but their London HQ.