As with any bike, wheel weight tends to be the main detractor – try a 650 with light wheels and they don’t have as much lag.
This is true in my experience. Not that I have any experience with 650b :-), but comparing 29ers with 26″ bikes absolute wheel weight does seem to be a major factor. I had a few demo rides on 29ers (with OEM wheels/tyres) and found them to be sluggish on steep climbs (the ones where I’m coughing up a lung just to keep moving). Then I built up my own 29er with wheels (Hope/Arch + Racing Ralph) that were actually a bit lighter than my 26″ set (Hope/Flow + Nobby Nic) and any sense of sluggishness was gone. Now winter has come and the Racing Ralph can’t cope I’ve put Nobby Nics on the 29er and it is feeling more sluggish on those steep climbs than the 26″ bike.
So, basically, lighter wheels are better, but we knew that anyway.