The quantity of items being added are likely going to be the main reason vs how other aspects are then trying to get the needed metadata, etc to be able to display the separators & other playlist items as needed. Also from what I could see in the gif, the lack of a reasonable wait between double-clicks most likely means the local library -> main playlist action is still in progress when the next attempted double-click is happening along with the attempted update of the now playing indicator (since most of this is done on various background threads & the timing interactions might be off with them with all of the really quick double-clicking happening).
The separator titles not completing being updated is definitely wrong which I'll have to look into. The other aspects not being able to react fast enough & that causing the lack of the expected now playing update, etc is something I'll also need to look into though it would be useful to see if the current build 20088 makes any difference or not as it had a small change to how metadata from the local library is obtained (not done directly with this thread's issue) which may or may not help what you're experiencing.
Hopefully that makes some sense as I think it's mostly down to triggering too many of the actions too soon & they're taking a bit too long to complete vs the rate of double-clicks being attempted.
-dro