Often, there is a much more trivial reason for slowing down a Zigbee network that has worked fine in the past and nothing was added or changed recently - the Zigbee channel that the mesh is using is jammed by a strong signal WiFi network. You can check and compare whether your home WiFi channel does not overlap the HE Zigbee channel. Or it may be a neighbour's new WiFi network...
You can read more here: Zigbee Channel Overlap
