Same thing here. It'll start at 0.02MB/s for about 20 seconds, and then I'll get the Unable to update patch files error [30143] and [20496]. I've reset the router, tried it without the router, I'm always on ethernet so it's not wifi, I set up all the forwarding ports in the router, I've reset Internet Explorer (which is what the client uses) to default, even though I've never touched it. I've made sure it's not in 'Read Only' mode as some have experienced in the past, I've disabled all anti-virus' on the computer, use a static IP, etc etc. I went ahead and updated a bunch of games I haven't played in awhile and each of them are updated fine, at normal speed.

From what I've read of other peoples accounts of this error over the years, this tends to happen a lot on big patch days. If it doesn't fix itself by tomorrow morning, I'll call my ISP.