I actually disagree with this. Jungle Diana is my favorite champ/role, so as soon as this patch dropped the first thing I did is test her clears and she is more than capable of clearing the new jungle quickly without taking too big of a hit to either her mana or her health. Here's a tip: Since the buffs spawn late enough that leashes are infeasible, start with your shield at gromp. Start by smiting gromp to get the mushroom smite reward, then pop your shield and go to town. The shield does a lot of damage and keeps you healthy, and the mushroom buff increases her already fast clears by a lot. Move on through the small camps and then come back for your blue as soon as your smite is back up. You should still have the mushroom buff, and have plenty of HP to kill blue. With this route you don't waste any time or experience and can clear quickly and efficiently while taking a minimal amount of damage.
Oh, Gromp? .___. I've been doing it wrong lol.
I've started with the Golems to get the Stun buff, then go to Red and then the little birdies before recalling.