If you're serious about learning this piece, sight read through it once, to get an overall feeling for it and see where the hard parts are. After that, take your time, learn one section at a time, and don't go on until you can play that much with almost no mistakes. If you want to bring it to performance level, expect to put in the time needed to learn it well.
As for what speed to sight read, really I think, mox nix. If you were trying to develop sight-reading skills for their own sake, I'd say keep a steady tempo and go slightly too fast. But if you're just exploring a new piece, do whatever feels comfortable.