Just for the sake of trying something else I put a tekComm 10/100Mb switch between the Mini and the router and changed the cable that goes into the Mac Mini. The performance is about the same.

I'm seeing a maximum of 950KB/sec over Samba locally, but it will sometimes just drop the connection and at times it will run much slower. Downloading frrom the internet the best result I've seen so far is 600KB/sec, but some downloads specifically will download much slower.

I tried receiving a DCC on IRC and it was below 100KB/sec and stalling to the point that the transfer could not be completed despite several tries. Receiving it on my shell I got around 4MB/second, so I should easily have maxed out my 20Mb connection on this.

In other words there's no real pattern to speak of, other than that all transfers are below what one can expect. Local networking, IRC transfers, FTP transfers and HTTP transfers all suffer to some extent and all drop the connection completely at times.

I may have another switch somewhere to test with, but I'm starting to think it may not help after all.
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edit:

I tried setting the envarc settings to 10Mb/sec and now I get between 600K and 950K on a file transfer from the local network. Internet transfers are up to 600K/sec. This is obviously much improved, but it's still not acting right. The transfer speed increases a lot at first, suddenly drops and slowly trains back up again (and this is frrom a server nearby with plenty of capacity)

[ Edited by Trezzer on 2009/10/30 11:55 ]

[ Edited by Trezzer on 2009/10/30 22:26 ]