yes and no
yes because i enabled the option and also forced quality over performance
yes because i start DT with these parameters: --configdir K:\config --cachedir K:\cache and K: is a 8GB RAMDISK
no because DT has a bug or the feature is missing: currently to have the full previews, you have to view the picture in full mode (hit F) and they are build as you browse, very slow.
no because I also created a script to pre-build them and pulled it to Github, but that works only while Darktable is off and sequentially, and is very slow as well.
bottom line, it's nowhere near as fast as LR and is not done in parallel of you working normally, therefore it's a real pain and practically useless function. I have no idea what the issue is because DT developers do not share a discord or IRC, there is no way to chat with them.