Originally Posted by
corrado33
They'll certainly slow you down
on the uphill 
Downhill not so much. Unless you were talking about in both directions.
My Farley 8 hardtail fatty weighs less than my specialized enduro. Gobs of traction with crazy cornering ability, sick lean over ability. As stated, it matches my dual sus in all but the rockiest of tech trails. On the flow and smooth, it's FASTER. Motor dependent of course. YRMV. I know a few riders who have sold their 29ers to ride fat year round.