Now here is something odd that's recently started...I don't recall it ever happening before beta 12, but it did sometimes. It only affects the viewport image, so I thought it may be a driver issue. But I rolled back a couple of versions, in addition to using both the latest 'certified' and beta versions of the Nvidia driver.
But, and here's the kicker...it doesn't affect the render.
Also it takes creating a new scene, without a hair node or a restart to fix.
When it is what I think, then this is something related to DAZ Studio itself. If this happens (again) go to "Preferences" -> "Interface" and then switch the "Display Optimization" to the next lower settings (I guess it's at "Best" currently, just turn it to "Better"). Finish your scene and turn it back to "Best" if you want, when it is set to "Better" the viewport is a little bit slower.