I have the same problem and tried everything previously mentioned in this thread.
Nothing has worked properly - the Insert Citation function is faster for a few minutes after closing and starting EndNote and Word again, but it gets slow again.
For now, I have found a very odd (can’t really explain why and how it works) way to make it work. Whenever i click “Insert Citation” and it loads for ages, I simply open a new tab on my computer via cmd+tab and instantly go back to word via the same cmd+tab - and for some weird reason the Insert Citation function has then opened.
It’s a bit of a hassle and I don’t know if it works for anyone else but me, but I thought I’d share it with you! At least it seems to be a preliminary fix for me.