maaku,

That's very interesting. Can you pinpoint the reason for this? Can you elaborate on your setup?

In theory(*), there shouldn't be a very high networking load at the moment.
Blocks are currently under 1MB, and there is less than one transaction per second. This implies sending 1MB once every 10 minutes, + around 1/2 KB per second for streaming transactions. All of this should be multiplied at most by the number of connections your client has, but in practice, most nodes should receive flooded messages and don't have to forward to others (Edit: in expectation, every node receives a message once, and so every node should in expectation only send each message out once). This shouldn't be too prohibitive...

There is additional load due to new nodes that come online and need to catch up and so may download many blocks at the same time. Perhaps this is the source of the trouble?
(Edit: I'm also ignoring inv messages and other small protocol messages above)

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(*) as they say: in theory, there is no difference between theory and practice, in practice, there is.