@Nutsnut 

Hey

Welcome to the world of WiFi and strange speeds    what you are experiencing is not uncommon sadly

The M4 model you are running is an AC (WiFi5) grade device which is specced to run at 866mbps over 5ghz wireless.   Sadly due to the nature of wireless technology, you will never get anything close to what it reads on the laptop / phone, in reality its around 40-60% of the stated speed in real world.  There is a number of reasons for this (refraction, delay, absorbion encryption overhead etc) but that is not import now.

As you are getting around 480mbps at the Main Deco, I would say that its running WiFi at full speed to give you that and that really is as fast as you are going to get.   The reason for the slower speed on the 2nd Deco when connected wirelessly is due to the overhead involved. 

As the main Deco has a MAX speed of around 480 (based on your testing), the second node will get half that again due to the extra "hop", so its max speed may be 240 at best.. add some distance and interference its likely around 100 which is what you are seeing.   Sadly this is the nature of Mesh..

Running the cable between the two allowed the second node to bypass the WiFi limitation and offer you the full speed via the Cable.   Superb idea.!    

However it could well be that  the channel your Deco is running on is also an issue.  In your Deco App under Advanced there is a "network Optimizer", run this and it will scan to see if a better channel is available and switch to it for you.  Hopefully should help!

Outside that you could try switching the Deco to Access Point Mode if you have a cable in place to the router.  This would offer you as fast as possible over the cables.

Ultimately, sorry to say at 500mbps internet speed the M4 is at its limit now, dont get me wrong its not gonna die but its at full capacity here.   If you want more speed you would need to move to the AX (WiFi6) range of Deco which offer a lot faster speeds over WiFi, but these come at a price premium.

  @Philbert Thank you for your response.

You seem to miss the part where i've ran an Ethernet back haul to the 2nd deco unit and manage to obtain good speeds both wired and wireless, but this only last 1-2 days before the deco needs to be restarted in order to obtain the same speeds. So this isn't an issue with the wireless overheads that are associated with mesh network.

I've tried all the netword optimisations etc and i'm already running the deco's in AP mode.

I've tried replacing the deco with the unit that is on the 2nd floor, but the same behavour is repeated, so something isn't right with the firmware, the speeds are there but the deco unit wont consistently hold them.

Edit - Even when connected wired, directly to the 2nd node via the switch or directly the speeds are still slow unless regularly restarted. So this is not a wifi issue.

  @Nutsnut 

Hey

You seem to miss the part where i've ran an Ethernet back haul to the 2nd deco unit and manage to obtain good speeds both wired and wireless, but this only last 1-2 days before the deco needs to be restarted in order to obtain the same speeds. So this isn't an issue with the wireless overheads that are associated with mesh network.

No I had not missed this part, I had actually commented on it saying it as good idea you had doing that.

Running the cable between the two allowed the second node to bypass the WiFi limitation and offer you the full speed via the Cable.   Superb idea.!  

You started the post indicating that there was slow speeds when using mesh, so you moved to wired.  I was only describing why it was slower via Mesh as not all users sometimes realise that mesh can be slower vs cable.

I've tried all the netword optimisations etc and i'm already running the deco's in AP mode.

Are both Deco cabled back to the ISP router? or is one going through the other on way?   Ideally these should have a straight connection to the ISP router for each Deco.

I've tried replacing the deco with the unit that is on the 2nd floor, but the same behavour is repeated, so something isn't right with the firmware, the speeds are there but the deco unit wont consistently hold them.

Thats quite a big jump assuming its firmware off the bat.   Have you ensured that your test device is connected at 5ghz when the speed is slow, would it have moved to a 2,4ghz connection?   2.4ghz is much slower than 5ghz and more in the speed range you are indicating.    Also what test device are you using?   Any Apple based devices are pretty bad for this issue and should be avoided when testing network speeds, they are notorious for showing the wrong results.

Furthermore how are you testing this speed?  Is it via something like speedtest or fast online?    If so that is not ruling out your WAN speed and therefore you cant say for certain it is the WiFi.   When its slow you are best testing the WiFi performance using something like IPERF3.   Run this on 2x devices (one LAN one WiFi) to see if the speeds are limited then. 

  @Philbert 

Hey

You seem to miss the part where i've ran an Ethernet back haul to the 2nd deco unit and manage to obtain good speeds both wired and wireless, but this only last 1-2 days before the deco needs to be restarted in order to obtain the same speeds. So this isn't an issue with the wireless overheads that are associated with mesh network.

No I had not missed this part, I had actually commented on it saying it as good idea you had doing that.

Running the cable between the two allowed the second node to bypass the WiFi limitation and offer you the full speed via the Cable.   Superb idea.!  

You started the post indicating that there was slow speeds when using mesh, so you moved to wired.  I was only describing why it was slower via Mesh as not all users sometimes realise that mesh can be slower vs cable.

I've done the speed tests directly wired to the 2nd node, via laptop and Xbox. Even when using wireless i'm getting 460-480 after a restart, but after 24hours its back to 140mbs.

I've tried all the netword optimisations etc and i'm already running the deco's in AP mode.

Are both Deco cabled back to the ISP router? or is one going through the other on way?   Ideally these should have a straight connection to the ISP router for each Deco.

My setp us is as follows. FTTP ONT > Ubiquiti Network 4 as router > Wired to Deco in office > This Deco is wired to 2nd deco down stairs.

I've tried replacing the deco with the unit that is on the 2nd floor, but the same behavour is repeated, so something isn't right with the firmware, the speeds are there but the deco unit wont consistently hold them.

Thats quite a big jump assuming its firmware off the bat.   Have you ensured that your test device is connected at 5ghz when the speed is slow, would it have moved to a 2,4ghz connection?   2.4ghz is much slower than 5ghz and more in the speed range you are indicating.    Also what test device are you using?   Any Apple based devices are pretty bad for this issue and should be avoided when testing network speeds, they are notorious for showing the wrong results.

Furthermore how are you testing this speed?  Is it via something like speedtest or fast online?    If so that is not ruling out your WAN speed and therefore you cant say for certain it is the WiFi.   When its slow you are best testing the WiFi performance using something like IPERF3.   Run this on 2x devices (one LAN one WiFi) to see if the speeds are limited then. 

I'm getting the same performance over LAN and Wired on the second deco, where it runs at the expected speed for 24hours, then drops unless restarted. Speed test via Ookla and also via file transfers over the network.

  @Nutsnut 

This Deco is wired to 2nd deco down stairs.

I would start with bypassing this and seeing if the issue disappears.  In AP mode its best practice to avoid daisychain on the Deco, with this setup you are using the Main Deco as a switch trunk port, which it is not designed to be.   You should rule that out first by going direct to your ubiquiti switch for each Deco, even if this means running a temporary cable for the testing.

See how that holds up and if you experience a speed drop again, then look at the switch itself or the patching.  

My gut feeling is its the daisychain that is causing it, you are using the Main Deco as a switch which is outside best practice for this hardware.  Deco is not like the Omada EAPs that have a dedicated Downlink AP port.