Zigbee latency - network saturation or poor link quality?

I’ve been having a few recent issues with Zigbee latency, particularly in the bedroom which is the further room from the Zigbee coordinator. I’m using the Texas Instruments Sonoff Dongle (firmware Z-Stack 20210708) with ZHA. The dongle is on a 3 meter shielded USB 2.0 extension cable. I’ve tried various placements but I’ve not seen much improvement in latency or link quality.

Lights get stuck between colour transitions states and smart sockets take several seconds to turn on after toggling the device in HA. All of my routers are Philips Hue or IKEA, light bulbs and smart sockets. Occasionally the most distance socket becomes unavailable completely despite other routers being a couple meters away in the same room. It does not become unpowered, just disconnected from Zigbee.

This is the visualisation of my network in ZHA. A decent number of my devices have green links between one another but the coordinator has no green links to anything except one lamp which is literally inches away from it and is currently offline while dealing with a warranty claim (another issue).

How do I determine whether these issues are caused by saturation (I am using Adaptive Lighting which might be causing excess traffic) or by poor link quality/interference? Is there a log I can check for all Zigbee traffic and determine a culprit? Thanks for any advice.

In general networks like Zigbee/Z-wave/Thread or other mesh networks often gets improved by more repeaters, which is most mains powered devices.
So but a few extras of those an mount them on the way to the bedroom, them wait a day and check your network overview again.

Many devices will update its mesh path over time.
Some will have to be rebooted and a few will have to be repaired (maybe with a factory reset first)

Make sure that you never power off any mains-power Zigbee Router device on purpose as Zigbee Router device are meant to always be available or else devices connected via them will loose connection. This is a common mistake people do with Zigbee lightbulbs as many connect then to dumb wall-switches which is a big no-no unless you specifically buy Zigbee lightbulbs that are Zigbee End Devices (non-routers).

Other than that you probably just need to add more always-on mains-power Zigbee Router devices and place them in strategic locations to get better coverage and range as well as improved redundancy.

Personally I would suggest buying three or more Sonoff ZBDongle-E USB adapters and converting them into dedicated alway-on Zigbee repeaters/extenders by flashing them with Zigbee Router firmware.

Recommend read and take actions to try to follow all the best practice tips here:

Thank you both, sorry for late response. I should have clarified in my original post but I do have a good number of routers and with the exception of the one faulty lightbulb, everything stays on at all times. Lights are controlled with buttons rather than mains power.

I do have one or two extra smart sockets that I don’t actually need for anything but could be added just as routers. We live in a flat and even the furthest room is only about 5 meters from the coordinator.

If I disconnect the coordinator for a few hours, will this force devices to forge new connections when it comes back online? I suspect some of the routes are not optimal.

Also, is there any logging to check the utilisation of the network? Can I be certain it’s just poor signal or could it be excessive traffic? Thanks again both