If you have not already then suggest upgrading to a modern Zigbee Coordinator like for example ITead’s “Sonoff ZBDongle-P (CC2652 based updated to latest firmware) as well as add at least a few "know good"dedicated Zigbee Router devices between the Zigbee Coordinator and any devices having issues.
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 do not need to make as many jumps and not as many messages have to be resent. And having a Zigbee Coordinator with better radio and antenna can obviosuly have the same benefits.
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).
Also recommend reading and following all the additional tips here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage