I'm currently filling a new WD My passport external harddrive connected to what I think is a USB 2.0 slot. (I found out my pc only has 2 2.0 slots and so I tested file transfer speed on all of them to determine which were 2.0) Only one slot operates faster than the others, which seems a bit odd, but that's besides the point. As I write, 73 GB of data is being transferred at 20 mb/s. This seems to be the highest it will go and it's fairly steady at 20 right now, but it can dip down to 10 mb/s. How good or bad is 15, say, for a usb 2.0?
In my experience USB 2.0 tops out at about 30MB/s in one direction. 15MB seems low, but it depends on if you're transfering large files (will go faster) or a bunch of small ones (will go slow).