I have a solid spray booth (larger metal Paasche) and it pulls through air and fumes pretty well. It vents out to the outside window just 2 feet away.

I've tried to use spray paints in the booth, but even with the booth fan on, there's a LOT of paint cloud that doesn't get sucked out. In fact, the fan first swirls the fume cloud around before it eventually pulls it out.

Any suggestions on how to improve the flow through to the outside?

Make sure you have a source of 'make up' air supply, which will pull air in to make up the air displaced by your fan.

I understand the core idea of your post, but I'm not sure I understand what to change...?

To push air outside there needs to be an easy way for air to move into the room, from outside. To move air you have to move (pull) all the air between the fan and some other outside source other than your ducts exit outside. The shorter the distance the better/greater the airflow. If the fan was at the exit to the exhaust duct work there would be less cavitation caused by the spinning blades so the air should flow more smoothly into the ductwork.

OPEN A WINDOW.

Better fan that moves a higher volume of air and at a faster rate.

Hard to tell exactly what to correct without a picture or two. But without the fan turned on, is there a natural airflow through the extraction vent? With the fan turned off, hold up a tissue to the vent and see maybe if your fan is fighting the vent to begin with. Hope this sounds clear.