Slow upload with Ethernet

I have a 27", Late 2013 iMac, with OS X El Capitan. I have a 5Mbps internet service and the download speed is OK, it's what's expected, but the upload is terrible. Nevertheless the Speed tests I've performed show it should be close to 5, like the download, but when actually uploading stuff, it's not. Like, 10 hours for a 400Mb file on Dropbox. I've tried uploading on Safari and Chrome, no good.

We have a Gigabit Ethernet and we are connected with Cat 6 cables. We also have PCs connected but they have no problems with the speed. The issue is only for the Macs (the other is a 21.5", Late 2012 iMac)

We are also connected to a Seagate NAS. The connection speed with it it's alright though.

Please help! We work designing and can't send big files to our suppliers and printers.

Thank you!