As I can't downgrade to 7.15.1, I am unable to post comparative query profile from that version. But looking at program performance, I have a suspicion that there has been a 10-30x slowdown in query processing. Please see this post for more background information which may help someone to reproduce the problem.
I have also observed kibana to be taking "slightly" longer in rendering saved searches (for discover) since the upgrade. Initially I dismissed this as misconception, but looking the query processing program performance, I am starting to feel the slowdown is real.