To do it in under 20 seconds, you'll need 4 players, and at least 2 ECM's. You'll need to wear suits, and having the aced "Transporter" skill will certainly help.

- A soon as the opening Bain rant is done, mask up on the spot.
-Plant an ECM on the floor, and run straight into the store.
-Throw the bare minimum number of bags out onto the street. When the mission timer gets to around 20 seconds, you'll need to plant the second ECM. While it's possible without this, the van often leaves as soon as the alarm sounds, and you don't want that happening.
-Move all the bags from the street and into the van
-Get outta there.

As the difficulty gets higher, you'll need more and more bags. This is why 4 players are essential.

On solo, expect to do a "Normal" in around 30 seconds. Anything above is not really possible without fully upgraded ECM's, and even then you can only do "Hard".