For cutting acrylic, the air pressure does not need to be high. It only need to be high enough to prevent fire. One way to prevent fire at the bottom is to put a piece of wet material like paper towel. The cutting speed can be higher if we peel off the protection film on the acrylic, but the surface finish may not be affected without the film.
When cutting at high speed, there are vertical lines left on the edge of the acrylic. To get flame polish finish on the edge, use low speed, and power needs to be high enough to melt those lines. If air is too strong, when acrylic turns back into solid, it will have "waves". The top of the cut edge will turn white if air is strong.
For engraving, cast acrylic give better contrast than extruded acrylic. If sharp lines are visible, we can adjust the focus to be off a few mm to make it smooth. A second pass out of focus can melt the lines to "smooth them out".
For thickness above 6mm, it is better to use a lens with focal length greater than 2" to cut. For a 2" lens, here are a few cutting speeds for reference: