@Benwood118 

The issue with Ethernet cable has been resolved. If you can get 200Mbps on WiFi speed test, that means Ethernet cable runs at gigabit speed. Devices negotiate only discrete speed values over the Ethernet: 10/100/1000Mbps.

Your old cable had 100Mbps, that was bottleneck. New cable must be on gigabit, so cable is no longer a bottleneck.

You run Deco mesh in Access Point mode. There are no settings in Deco app in that mode to limit/throttle speeds.

Satellite Deco will deliver half of the speed of Main Deco, even in ideal conditions, less if conditions are not ideal. This is not Deco mesh issue, it is limitation of WiFi mesh technology.

Your Main Deco is capable of delivering 500Mbps, which is speed on your Internet link. Because it does not, the reason could be how speed is tested, or very high WiFi interference from household appliances and other WiFi networks.

Try speed test the following way:

1. Run Network Optimization in Deco app, agree to change WiFi channels if it offers it;

2. Keep only 5GHz band enabled for Main Network in Deco app:

You can change that back after you finished with this speed testing.

3. Power off all Satellite Deco, leave just Main Deco running;

4. Bring device you use to run speed test to about 1-2 meters (3-6 feet) from Main Deco, in direct view of it;

5. Turn device WiFi off, then turn it on. Check what does device report for associated link speed: How to check the associated/link speed on a wireless client. If it is less than 800Mbps, move it even closer to Main Deco. Ideally, it should be 866Mbps download and 866Mbps upload.

6. Run multiple speed tests back to back, using OOkla app, and write down best results.

Let me know what is the best you've got with speed testing in this setup.

  @Alexandre. 

First of all thank you for all your help! 

so did what you said and the main deco is fine 

however the other decos are doing about 120 to 150 max. so there might be interference with them. 

I have rebooted them and has improved a bit but not 400. 

  @Benwood118 

Satellite Deco will deliver half of the speed on Main Deco, or even less. This is limitation of WiFi mesh technology.

You have 400-450Mbps on Main Deco, that means you should expect up to 200-225Mbps on Satellite Deco.

One last thing I can recommend you to check. In Deco app, for each of Satellite Deco, open its info page and see what it says for Signal, example:

If Signal is Weak or Fair, moving Satellite Deco closer to Main Deco should increase WiFi speeds from Satellite Deco. If Signal is Strong, moving Satellite Deco is not necessary, it is the best place for it.