Adding at least a few "know good"dedicated Zigbee Router devices between the Zigbee Coordinator and any devices having issues should make a huge difference.
Having a few “great” Zigbee Routers (a.k.a. Zigbee signal repeaters / Zigbee range extenders) with better radios and antennas for better reception can affect performance positively as communication signals if messages do not have to be resent. And having a Zigbee Coordinator with good radio and antenna away from any EMF sources can obviosuly have the similar benefits, but Zigbee signals have poor range and worse wall penetration power so routers are a must.
To get the best suggest Zigbee Router devices today buying a few of ITead’s Sonoff ZBDongle-E (based on EFR32MG21) and flashing with Zigbee Router firmware then power them via USB-changers to work as stand-alone devices, (or alternatively for an out-of-the-box product buy a few of IKEA’s Trådfri Signal Repeater devices and/or Aeotec’s Range Extender Zi devices).
Again, highly recommend reading and following all the additional tips here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Home Assistant posted a video to clearly show Zigbee’s symptoms with electromagnetic interference:
Network optimization
You have to understand and remember that each Zigbee device by itself has very limited coverage, short range and their weak signals have poor wall penetration, so to workaround this a Zigbee network relies on mesh networking (a type of network topology/technology/architecture), which means that a Zigbee network heavily depends on having a swarm of mains-powered devices are a “Zigbee Router” that are always-on so they can act as a signal repeater and range extended by transmitting data over long distances by passing data messages through the Zigbee network mesh of intermediate devices to reach more distant Zigbee devices.
Thus the key to a great and healthy Zigbee network is to add/have many “Zigbee Router” devices relatively close to each other (and always powered on) in order for the Zigbee network as a whole to get good coverage and range. So often you more or less just need to add/have several mains-powered Zigbee devices in the network that act as “Zigbee Router” devices to achieve a stable Zigbee network.
