I am using s3cmd to upload to S3:

# s3cmd put 1gb.bin s3://my-bucket/1gb.bin
1gb.bin -> s3://my-bucket/1gb.bin  [1 of 1]
  366706688 of 1073741824    34% in  371s   963.22 kB/s

I am uploading from Linode, which has an outgoing bandwidth cap of 50 Mb/s according to support (roughly 6 MB/s).

Why am I getting such slow upload speeds to S3, and how can I improve them?

Update:

Uploading the same file via SCP to an m1.medium EC2 instance (SCP from my Linode to the instance's EBS drive) gives about 44 Mb/s according to iftop (any compression done by the cipher is not a factor).

Traceroute: Here's a traceroute to the server it's uploading to (according to tcpdump).

# traceroute s3-1-w.amazonaws.com.
traceroute to s3-1-w.amazonaws.com. (72.21.194.32), 30 hops max, 60 byte packets
 1  207.99.1.13 (207.99.1.13)  0.635 ms  0.743 ms  0.723 ms
 2  207.99.53.41 (207.99.53.41)  0.683 ms  0.865 ms  0.915 ms
 3  vlan801.tbr1.mmu.nac.net (209.123.10.9)  0.397 ms  0.541 ms  0.527 ms
 4  0.e1-1.tbr1.tl9.nac.net (209.123.10.102)  1.400 ms  1.481 ms  1.508 ms
 5  0.gi-0-0-0.pr1.tl9.nac.net (209.123.11.62)  1.602 ms  1.677 ms  1.699 ms
 6  equinix02-iad2.amazon.com (206.223.115.35)  9.393 ms  8.925 ms  8.900 ms
 7  72.21.220.41 (72.21.220.41)  32.610 ms  9.812 ms  9.789 ms
 8  72.21.222.141 (72.21.222.141)  9.519 ms  9.439 ms  9.443 ms
 9  72.21.218.3 (72.21.218.3)  10.245 ms  10.202 ms  10.154 ms
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The latency looks reasonable, at least until the server stopped responding to ping requests.