Setting “maxhairdepth” to 0 did the trick. Render times are now down to around three-four minutes, and that’s with GI on.
Of course the times will increase when SSS is added to the skin, but still.
I’m not sure the “oldschool” hair shaders are faster. Moreover, those won’t work in a path-tracing scenario (because they predate it).
Lighting for the scene: a path-traced area light (I rotated it for a more interesting angle) and DelightGI (the hair shader picks the HDR map from it automagically). Render times are coded as the last digits of the file name (min-sec).
You can see that to get lighter hair, not only you need to use a lighter colour (which is then inverted in the shader to get absorption), but also dial down the hair shadow opacity (right now I’m just using Os, the primitive opacity; the 3DL devs use an absorption-related calculation for transmission ray hits, but I’m not sure it’s worth it in a hobbyist scenario; hair casting coloured shadows looks neat in a non-GI render, but I have never seen anything like that IRL… okay maybe I need to find a pink-tinted bleached blonde girlfriend to perform experiments on, but hey). Any opacity tricks will slow the render down somewhat, but not into the forty minutes realm.
I varied the roughness values around somewhat for these renders. In theory, they all should be the same for all three lobes, but artistically, I think that TT being the most rough (wide), R coming next and TRT being the sharpest makes the most sense. Or maybe R and TRT should be switched (R is the uncoloured one, TRT is the glint-y coloured highlight).
There are also weight controls for the lobes, and the renderer will automagically normalise the values if they’re over 1, so as to avoid energy amplification.
I really like the balance between possibilities and complexity that the 3Delight developers are giving us.