One of the big factors for how smooth something feels is frame pacing, as opposed to raw frame rate. 60fps with steady frame pacing can feel smoother than 120fps with unsteady frame pacing.
If your hardware is struggling (ie, its capping out at ~40 fps) then there’s a good chance those frames aren’t coming in at a steady pace. The frame generation tech can only guess when the next frame will come in, and try to insert a frame half way through, but if that next frame comes early or late you end up with unsteady frame pacing anyway!
I’m not sure if you can cap to 30fps and then use frame generation to get you to 60 - that would certainly be a way to get steadier frame pacing.