As this isn't a Linux issue you will get better results reporting this in the regular Steam forums.
Considering this issue has already been assigned to a specific person and noticed by another, whereas I can only think of one time in the last year where I saw a Valve employee post on the forums, I rather doubt that.
I believe I've mentioned this before, but what we really need is another repository on here for reporting and tracking bugs with the client in general. It's the most commonly used of all your products, yet it's the only one that lacks a public bug tracker.
The January 7 client update may have resolved this by accident. (I say "by accident" because there doesn't seem to be anything in the patch notes referring to it.) I'll have to check again when there's another sizable update to one of my Linux games, just to be sure, but I haven't noticed an uneven download speed or impossible maximum speed since then. If this is an unexpected change, you may wish to make a note of it.
I've used the Linux client enough now to conclude that this issue is now gone, however that may have happened. I'm closing this.
Nothing extracted yet.
This is a non-platform-specific issue, but it's a pretty major one.
The speed that Steam claims to be downloading at seems to be based on something other than the actual download rate, like maybe the transfer rate between RAM and the hard drive or something, because it's constantly wavering between nothing and several times faster than my actual connection can handle. I'm on a roughly 140 KB/s connection, and the download screen has reported as high as 550 KB/s at peak. Meanwhile, if I pull up a network traffic monitor, it tells me that Steam is pulling down a near-constant 140 KB/s even when it claims to be "Working..." and not actually downloading any data.
This applies to the download rate reported by the downloads screen as well as the taskbar icon's tool tip (in Windows).