I’d echo what taxi man says
I’ve got a light 29er mtb and compared to that, of road my cross bike is rubbish
If you set up a rigid 29er with some very light thin tubless tyres (ie bontrager X0 1.9) and a longish stem and short bars. Then its a million times better off road than a cyclocross but not that much slower on road. It will be slower on the road obviously but not as much as you think.
Ok this is a ride I did from Twyford to Swinely on my rigid 29er. I managed to average 15.7mph on the road section to Swinley and that was with my 2.25 Racing Ralphs on.
http://app.strava.com/activities/232088852/segments/5649352500
over half an hour I can normally average 19-20 mph on 25mm road tyres on that route-ish.
The thing with the 29er rigid is that on some single sections I’ve actually got my best times (ie labrinth at Swinely) over the Full Suss.(although if I try to do the full trail with out stopping my times are quite alot slower on the 29er than my Full Sus as I tend to tire much quicker)