The airflow can be significantly better than a purple days stem, or about the same.
An clean PD stem and an empty solo tube have pretty much the same draw. But once you pack the PD, it has a lot more drag than the solo because it's narrower, longer bowl. The screen also slowly loses airflow to clog and requires fairly frequent cleaning. The screenless two hole design does not lose any airflow unless you pack a leaf down directly over one of the holes, in which case you would then unpack and repack the bowl more loosely. I say the solo is less restrictive. It has to do with it being one big hole with a screen instead of two small ones without, it's a wider bowl all together. With a screen in the bowl the draw is like that of an unpacked purple days, and is actually pretty comfortable to milk a bong fast with. Without a screen, or with some bad luck, if one or both of the holes get nugs stuck in them it can be uncomfortable.
Overall, it's usually as good as an empty clean purple days stem, and a screen in the bowl is a good idea for airflow and efficiency.
The solo and the purple days feel very similar to hit, but the solo feels it's skinnier, battery powered, harder hitting, high tech cousin. I especially find the taste to be stronger out of the solo, likely due to a less restricted airflow, bigger clouds and less of a wood and buzz butter aroma mixed in. You can push the screen farther forward in the solo to get very similar efficiency out of the two