Possibly lawn mower syndrome.

You start a 5 cylinder white engine and only run it for a short time,

Like if you move the car to get your lawn mower out of the garage.

Back in the 850 days, this was a frequent post.

It happened to me, I moved my car to the shade when I was waxing it. I had it towed to a dealer - they got it started, warmed up and then changed the oil and put in new plugs. (Plug fouling and oil dilution)

After that, a family rule was never start an 850 unless you are going to warm it up fully.

The car may actually crank faster than normal because some of the valves are stuck open.

My theory - When an aluminum engine runs for a very short time, the only things that get hot are the exhaust valves - they stick in the valve guides.

Holding the pedal to the floor during starting shuts off fuel flow, and the engine clears itself.

Crank a little longer than normal, pause, crank a little more than normal, pause ... repeat - Keep the gas pedal on the floor.

Twenty seconds sounds like the right time. Get your repairs done before you go chasing windmills.