I wonder how you notice that the shadow files are not being used?
Thoughts:
If the timeline editing is (still) slow, does it help to downscale the quality of the preview screen and the display options?
When I test the switching
off and on
of the shawdow file creation, while editing, I see that PD will duly produce shadow files (when done the the marker in the bottom left corner goes from yellow to green.)
In "normal" editing situations it takes a while before (all) shadow files are available, even when the timeline is being filled up by the users editing. So this would be normal behavior and this would be something developers would take into account?
So, if besides the green marker there is no way telling whether shadow files are being used (if that should matter at all), maybe ask the developers / customer support what really is happening behind the scenes.
Saving the project and then reloading it again (after having switched on the shawdow file option and the markers are green, I think taht PD will rescan the media room and likely also look at the availaility of shadow files. Would that help?