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On a job site I maintain (IT admin) I notice that downloads start at a pretty high rate (cca 700MBps) and then drop significantly to rates as low as 40Kbps... Not sure where to look first to determine what is causing this. My first suspect is the SonicWall TZ 210, but haven't found anything obvious in the settings yet.

Basically, when downloading stuff, from any source, if you keep pausing and resuming the download, you can keep the speed up in the 700-500MBps range. But that is pretty ridiculous, I say.

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Are any proxy functions taking place on the SonicWall, such as content inspection/virus scanning/blahblah? Are there any QoS policies in use anywhere on the network (are the switches shared with phones for instance)? How's your CPU and RAM utilization on the Sonicwall? Which firewall policies touch the packets? What happens if you put a computer where the ISP would be, set the interface on the firewall to a static IP, set the computer to an ip on that subnet, allow all (type policies), and then use iperf? – mbrownnyc Sep 22 '11 at 18:09
Also, web browsers some times will buffer the file you're downloading, so this may be why you see this fast speed after pausing... for instance, firefox downloads a file to a temporary location while you're selecting the final save path. Watch your syntax also, MBps = megabytes per seconds... what you mean is Mbps (megabits per second) (useful to know when converting values in Google for instance). – mbrownnyc Sep 22 '11 at 18:21
I've seen the same thing with a SonicWall firewall. Any chance of temporarily swapping it with another make/model firewall to rule it out as the issue? – joeqwerty Sep 22 '11 at 18:39
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