Udo Rader wrote:

On Fri, 2007-10-19 at 08:46 +0100, Ed W wrote:

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I've verified this with various customers and thus I am completely lost here. The only service affected with this massive slowdown seems to be dovecot but unfortunately I cannot think of a way to debug the issue ...

Duplex mismatch on the network card?

grrrrrrrrrrrrrr, yes!!! Checking the link status of the NIC it showed that indeed duplexing was giving wrong results. Forcing it into the correct mode did not change anything, but finally replacing the card with a new one solved the riddle!

Really slow mysterious speeds are nearly always due to massive packet loss and 9/10 times this is a duplex mismatch or failing network card.
Curious huh...

Ed W