So, in my case it appeared to be a problem with the “mode list” of the “fan” type device, as part of the Home Assistant community app. Other device types don’t seem to be affected;
With every update of the “fan” device, all related flow cards were re-created. This slows things down massively, when these updates occur every few seconds.
Jeroen Wienk (Athom) spotted the continuous re-creation of flow cards (after he was allowed to access my Homey);
With that information, developer Ronny found the culprit in the end, and pushed version 1.15.2 of the app, which solves the issue!!
