I wouldn't change the wheel size-too expensive- I would literally get smaller diameter tires-generally smaller diameter means shorter sidewalls-lower stand over
It is disc-so you could go to 26" wheels-no they wouldn't be "SLOWER" actually they would accelerate a bit faster-but smaller wheels are slightly less stable
But 26 vs 29-nothing to do with actual speed-and the smaller diameter wheels should allow you to accelerate a bit faster
26 should allow quicker handling-but at the cost of stability.
Folks have done just fine with 26" wheels-so not as if they were falling all over the place
29-700c marketing gimmick of course-no one here is a pro rider-so 26 vs 29-doesn't really matter much-650 same story-gimmick making a comeback