I would say that something between 7.12 and 7.15 was changed and Kibana, but I did not have the time to look into all the release notes.
I had some problems with Kibana 7.15, including a discover bug, that made me postpone the upgrade until the bug is fixed, maybe there is a related issue about this slowness you are getting, have you looked into the github issues to see if it was reported?