Only real comment I have is that I had to return a new harddrive a few years ago, that was practically new, no errors kicking back from it, it just had a slow r/w speed. I mean real slow, something like the speeds ur talking about.
Scan took it, tested it, agreed and replaced it. So I have no reason as to why. Just happens I guess.
Whether it's the same in ur case, I dunno. I'd suggest taking all the data off it and running simple tests. Test u know for sure should give a level of result, like hdtach or something. If it's knackered, there's not an awful lot u can do about it.