Obviously I can't say what'll be fastest for you, but I have a Garneau P-09 -- which apparently tests really well for many people in the wind tunnel. The ear flaps pop outward, so it's easy/quick to jam it on your head in transition. Really nice helmet, I use it with the vent plug out and never have had a problem with overheating.

X2 on the P-09. On me it field tested faster than the Giro Aerohead (I wear the P-09 without the visor), and was also much easier and faster to get on and off.

Also, as everyone else piles on to the next great thing, you can get some pretty good deals for the P-09.

The Aerohead functions just like a regular road helmet. Pull it on. Buckey the strap and go. You can pull the visor off the top and pop it on at your convenience.

I've got a Giro Met Drone. It's pretty quick and easy to get on. Especially now they have magnet fittings for the visor - before it was a pain in the arse as the visor would inevitably pop off in the transition bag and you'd have to waste valuable seconds clipping it back on.

Aerohead.

Since the visor covers your ears, you can put it on with the visor up and you don’t have to pull the helmet over your ears, so it goes on as easily as a road helmet. You also won’t have the straps get tangled around the ear flaps.

TBH, the savings is trivial though.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)