Adding Chris so he is aware of this and can, maybe, make an API test tool for chunked uploading available for people to use when they run into the problem to see if the problem is browser-dependent.

I saw a similar issue between FF on Windows 7 and Ubuntu: On windows a 200 MB upload was taking about an hour, and took 10 min on another machine directly after the first upload.

I'm uploading a 200 MiB ogv to test bug 36587 (this: https://archive.org/details/wikimania2008_Communication-issues-musings-of-a-dinosaur ) and it's still very slow.
With Firefox, I was given an estimate of 40 min, it never went above 230 KiB/s according to nethogs and it oscillated a lot (in 1-230 interval, mostly around 100); with Chromium, the max is the same but it oscillates less so the average is faster and I was given an estimate of 15 min. I'm still uploading and I didn't take the chronometer so I can't say how accurate the estimate is.

I could not verify for certain that its a problem of UW or not, but looking at the code I don't think UW is doing anything wrong here.