@MStraeten , I recently downloaded darktable 4 and noticed on my m1 Mac that the GPU was not being used. Also, in settings the OpenCL options are all showing as not activated.
Please check this thread Darktable 4-MacOS M1+OpenCL - #10 by hannoschwalm
Did you notice this issue? Some person at the end said that OpenCL was never used but I am sure I have seen it used on 3.8. I had also tuned the parameters so I clearly remember.