As much as I dislike them, I have been doing some 29er testing of my own recently. Results – meh.
On some short trails on a local loop myself and another rider swapped between a brand new 120mm full sus 29er, and my zesty (with forks set at 120mm) both with comparable tyres. On a rolling start there was little to separate the two bikes. The 29er was grippier and more predictable, but not faster. The zesty “felt” much faster, and much more fun.
From a standing start the 29er got smoked every time. I was expecting it to accelerate slower but not that much slower. Throwing in some jumps on the trail though really left it floundering as it was nigh on impossible to get the front wheel up. On the climbs and on flatter less twisty trails the 29er was less work, it seemed to need less rider input, perhaps less involving but again not discernibly much faster if any. Maybe over much longer trails that might manifest in quicker times as riders stay fresher.