Mostly needle size.
I have three Iwatas, ranging from the AH at 2mm up to the TH at 5mm. I always used them for the size of the model, not the paint an project. So when I went to prime the 1/35 heads, I grabbed the small HP-AH for the quick color change. Very thin Vallejo ploy primer, shot it light, very wet coats like I've always been doing. When I went to paint, I saw the texture of the figure. It looked like it had measles on top of hives while sitting in a pool of ice water. I've heard people complain in the past regarding those primers being 'pebbly' and I even ordered the ridiculously expensive Mission Models primer to use on these. But while waiting on it, I looked into the problem and it was mainly nozzle size. Yes, size does matter. I was even thinking about a Harder and Steenback 2 in 1 CR brush. But I tested a few methods and using the giant HP-TH without the color cup, it's as quick a change as the AH and the 5mm needle passes a very fast wet coat, and at about 8-10 PSI, no pebbles.
I also skipped primer on a few tests and on faces where you're not going to be handling it, if you gi
Another trick I'm sure everyone knew but me, to never take your finger off the air, let the needle seat again after spraying and use the air to dry the paint before adding more. Plus it is supposed to clear the chamber lessening the chance of paint buildup.
Although I'm still eyeballing a H&S.