Steam does a lousy job of bandwidth throttling. I had it set to limit downloads to 512kbytes/sec. (Why? Because despite using a fancy router with QoS control, QoS still sucks in 2014. Can’t stream video while downloading, derp). Here’s the graph of throughput from my router. X axis scale is 1 pixel = 2 seconds, max throughput is about 900 kbytes/sec. I turned off throttling halfway through.
So looking at the first half of that graph, what Steam is actually doing is running at full capacity for about 10 seconds, then backing off to the requested 512 kbytes/sec for about 10 seconds, then averaging it out by going slow for 10 seconds. Back and forth. That’s just awesome.
I sure wish QoS worked on the modern Internet. The academic research I’m familiar with was all worried about solving the hard problem of fair congestion management through the whole Internet. I don’t care about that; I just want to manage QoS on one link, from my router to my ISP over the slow pipe.