Try that. Why is noted at https://community.smartthings.com/t/faq-networking-and-reducing-channel-interference-between-wifi-and-zigbee/40159 and is a little technical so I defer to the article but would try the router WiFi channel change.
I recently installed Hue bulbs in my living room and the required bridge is plugged into my Airport Extreme. Hue controls my lights just fine. But since the install, my network speeds have plummeted to border line non-functional. (From 120mb/s to 5mb/s)
I've found that if I simply unplug the Hue bridge from the router, my speeds return to normal. Any idea what the deal is? Is it the bridge? A router configuration issue?