tl;dr: On one computer, slow network connection consumes the entire bandwidth, affecting both internal wifi adapter and when browsing through a phone modem.
comment: I posted this issue on Super User several hours ago without any reply.
I have Ubuntu (13.10 and previously 13.04) and recently (roughly for two months now) download rates are very low, about ten percent of my connection. On the other machine (elementaryOS) The download rate is fine. My common test is installing packages. At first I thought this was a wrong mirror issue, but I get the same rate when downloading podcasts (mostly from podiobooks) and isos from Ubuntu servers. Torrents are downloaded fine.
What is bothering me even more, is that even at this low download rate, the wifi is clogged. This morning, I downloaded podcasts episodes (again 10 percent of the bandwidth) on the problematic machine, and I couldn't even watch a YouTube video on the other one. I have a 5MB/sec connection which is low, but it should be sufficient.
Even copying files over the wifi from one machine to the other hogs the network.
Several days ago I replaced the DNS servers on the router from Google's to OpenDNS. This helped for a couple of days but now the machine is back to its bad behavior.
At first I thought that this could be an issue with my wifi card, but this time I connected my phone through USB and used its wifi with the same results.
I should note that i disabled ipv6.
So, how do i diagnose this, where could the problem reside?
** Edit: **
lspci -nn | grep 0280 03:00.0 Network controller : Realtek Semiconductor Co., Ltd. RTL8188CE 802.11b/g/n WiFi Adapter [10ec:8176] (rev 01)
Also, I think that the main event that triggered the drop in performance is that when I'm at the office, I connect the laptop to an external screen and keyboard and I close the lid.