He's actually a very good skater, but the issue seems like you do not understand what kind of dman Seider is. If you were expecting end-to-end ice rushes and flashes with lots of offense, then he's clearly not that guy. But if you were expecting a workhorse that has amazing positioning and defensive play along with a physical shutdown game and some offensive flair, then Seider has been exactly that.
Seider's value is in shutting down opponents, not in getting offense. Oddly enough, he's leading all d-men in points so far, so there goes that.