Hmm... Sometimes certain parts can get real slow like that and take ages to finish. I just had a quick test with the latest Gaki episode on Youku, mostly the parts were fluctuating between 80kb - 200kb. Later they started hanging and dropped to 20kb for awhile. Then they picked back up again to around 60 - 120kb. With some dropping to 40kb - 60kb for awhile. Then the very last part got down to 20kb for a bit before picking up to 150 - 280kb then down to 40kb - 80kb. I think it just varies a lot, but overall, aside from the very last part, didn't take too long to download due to the moments where it picks up speed.
I'm assuming you're talking about a video on Youku.com, if it's Tudou.com it's not surprising, their servers are ridiculously slow to download from...
By the way, I sent you a private message awhile back about downloading from Youku with JDownloader using WantYoutube to get the links regardless of the "Mainland China Only" changes.
There's also apparently a browser addon called Unblock Youku, dunno if that helps to get JDownloader to capture Youku links again, haven't tried yet.