Speed of donning is really a function of regular practice... and motivation. Less about design.
Once upon a time in Baghdad's Green Zone, I could habitually wake from deep sleep, roll off my cot, and don a completely kitted out Eagle Maritime CIRAS (full up soft & plate armor chassis), ACH, ballistic glasses, and get proned out... in well under 29 seconds. Which was about the maximum counter-mortar radar/Giant Voice siren warning to impact time for incoming rockets & mortars. I got to where I could accomplish that task swiftly, routinely, & consistently. My T-wall surrounded CHU (sleeping hut) was too far from the nearest concrete bomb shelter to make a sprint ... so I'd just hug the floor and wait for the crump or blast of impacts. While on the floor, I'd fasten bedside Teva sandals to feet, don Nomex flight gloves, and then reach up to secure my pistol (kept under my pillow). Long gun already laid out on floor alongside cot. NODs, ammo, water, IFAK, etc. already attached to CIRAS. Weapons not relevant for an indirect fire attack, but I liked to get them to hand in case I needed to leave & go outside. Everything in its place, grabbed in order of need (armor first, helmet second, clear ballistic glasses third, etc.), and no fumbling in the dark for other bullshit like small arms, footwear, cell phone, ID, etc.
That big armor rig sat carefully positioned on the floor next to my cot. Everything needed for "Whump in the Night" pre-placed for handy access... and in order of priority for fast donning. I simply lifted that heavy rig up and over my head/shoulders... and slapped the open side of Velcro strap haphazardly closed.
My point is that any rig (soft armor, plate carrier, full chassis) can be donned very rapidly as long as it's close at hand, pre-positioned for immediate and unencumbered grasping, and you have periodically practiced donning it. Today I keep soft armor and a loaded duty belt hanging inside of the bedroom closet. I can don those pretty quickly too. One side closure of the armor stored always fastened, the other left open. Over my head, drop, and a quick sweep of the open-side's Velcro closure strap to secure the rig snugly.