Why would it look like a rock? And if it was sent, why wasn't it closer? Might as well claim that God put it there. If it actually was aliens it's quite likely that it wasn't sent specifically for us.
Here is a lot more information. It accelerated when it got close to the sun and then decelerated. Seems like a natural phenomenon of being pushed by strong radiation from the sun.
It entered the solar system at a speed of around 23km/s. That's painfully slow to actually travel through space and bridge any kind of distance in a reasonable amount of time. You need hundreds of years to get through the solar system at that speed let alone get to it from somewhere else.
Errr.. If it really was an spaceship, what makes you think this would be its top speed? I mean, I'm not saying that it was indeed an spaceship. But if it was, then I think it is safe to assume that it would be able to accelerate and decelerate at will.