@pk5dark Thank you! My Windows machine was indeed using an integrated Intel graphics card that I didn’t know I had.
I’ve not figured out the exact details, yet, but at least some operations are now much faster on the Windows machine.
I just compiled a version of Darktable without the little preview window in the top left. And it is indeed about twice as fast in all GUI operations.
(Add a return; immediately after entering dt_dev_process_preview_job in src/develop/develop.c. This was surprisingly easy to figure out.)
Still not as fast as Capture One, but getting there. Most operations now take less than 200 ms.