Gigabit internet, download is slower than upload on all ethernet connected Macs in the house (windows PCs are fine)

I am aware of all of this. And as I mentioned everything is cable with proper 5e ethernet cable. And I was using the same cable to perform all tests with all the different computers. It's not the cable. And the hardware definitely show me 1000baseT full duplex and flow control. I have tried to activate the jumbo frame (MTU at 9000) but it changes nothing.

And again, the upload connection is working full speed over the same cable, dongles (whenever I use one), etc. It's just the download that has an issue. I can only think that OSX is somehow limiting the download speed but not the upload speed (internal firewall ? traffic scan or whatever ?) I have disabled every app on the devices that I could think of, but nothing changes.

And the windows machine is reaching speed both ways no pb.

Nope. And please remember it happens on all 3 machines. Same limitation of download to a few Mbps.

And before performing the tests, I kill all possible suspect (time machine backup, iphoto, dropbox, etc.)

And the macmini (which as I use a small server) has none of that configured anyways.

I did not find anything wrong.

No processes are using network downloads while I am performing the tests.

And my cables are fine, they are brand new cat 5e cable (just moved in into this apartment and purchased all the cable like a month ago), and again, the upload speed is fine (full gig). And more than this I can do transfers from the Mac mini to the Mac Pro via local network at ~80 MB/s sustained.

So this tells me that the ethernet cards/cable/configuration are fine on the Macs. It is really just the download speed coming from Internet that somehow seems to be limited.

While performing the speedtest download, the download will plateau at around ~30MB/s while the upload reaches the full ~80MB/s (coming from activity monitor).

I put it back to its default (1500 MTU).

And to clear any possible doubt, these are the speed I am getting with my wife's work laptop using windows 10:

So the windows machine reaches the full 1Gbps. And I have not configured it to use jumbo frames either. It's completely vanilla configuration.

Just checked the modem configuration... it uses an MTU of 1492, so it is not even using jumbo frames...

And maybe there is something that I don't understand, but why would only the download be impacted by the MTU, shouldn't it impact both ways the same way ?

Just tried to set the MAC with an MTU of 1492, it changes nothing. And of course I also tried to configure the modem to use 1500 MTU, changes nothing either.

Alex

I do not. Also tried with a fresh new install of El capitam from a usb flash drive, same results

The other way around (writing on the Mac Pro from the Mac mini) the network can sustain even higher speed (probably the hard drive of the Macmini that's getting oldish is a bottle neck here):

(deleted)

Not a single one...

Yes it is very weird.

And the same speed "block" is happening on all 3 of my Macs (2 on high Sierra and one on Yosemite).

And this is what I get on any of my Macs:

On the modem and the Mac ?

Ok, because on the modem it is not allowing me:

MTU Size "8182" is out of range [1-1500].

Tried it on the Mac, same results.