@Albinj 

I am assuming your Satellite Deco X60 is wirelessly connected to Main Deco. With that, what you see is expected result. To find more about why is this, Google for "why WiFi range extender cuts bandwidth by half"

  @Albinj 

What speeds are you getting?

As mentioned above, the hop node will reduce the performance by half each time.  Sadly WiFi is not bi-directional, namely it can only send OR receive at one time so every hop adds an additional delay and this in turn affects throughput.  

What speed is your internet access and what speeds are you seeing?    Moving the node at your computer closer might help!

  @Alexandre.  Its the same deco so it should´t 

  @Albinj 

Hey

You mention it is the same deco, but you earlier posted that   " When I move to the host deco I get full speed " which indicated you had at least 2 Deco nodes.

Can you describe the setup you have in more detail?   Sadly we dont have enough info here to help you and its not clear how you are testing this, sorry!

Feel free to include pics as they always help!

  @Philbert 

my current setup is host node downstairs. Then a deco upstairs and that connects to the deco in my room which is the deco that is con etched via Ethernet to my pc

  @Albinj 

Thanks that helps, so yes it is a dual deco setup and that does go back to what @Alexandre. mentioned earlier

As mentioned earlier WiFi is not bi-directional, namely it can only talk to one device at one time.  The short reason for this is airspace collision, if 2 signals are received on the same frequency at the same time, then they collide and become jumbled together and this causes corruption, therefore in WiFi everyone waits for a turn to talk.   Hopefully that makes sense!

So in your setup, when testing on the PC for example, the ONLY device it is talking too over the WiFi is the other node (downstairs), therefore downstairs gets 100% of the airspace and time slots to work with.  Therefore you get 500mbps of speeds on the WiFi, its sent 500 over the cable to the PC, as these are separate connections, there is no waiting. 

Now on the iPhone, you now have 2 devices on the WiFi (the downstairs node and the phone), therefore each will take turns to send/receive. 

So data comes from the internet into the main deco, then over the air at 500mbps to the second deco.. while this happens the iphone must wait.  Then the 2nd Deco sends it to the iphone at 500mbps, main deco is now waiting.   

However, its not actually 500mbps as it took twice as long to get there, you have to include for waiting, this means it took twice as long so its actually 50% speed when you look at it overall.  Hence 250ish in the speed tests. 

Yes that is a VERY oversimplified answer but hopefully that all made sense?

OK so how to fix it.  

1. WiFi speed, if possible move the 2nd deco closer to the main deco and this will increase the throughput between them.  Therefore you are now 50% of 866 and not 500, that would help massively.

2. Ensure you are on the 5ghz connection, if you are on 2.4 it will limit you massively.

3.   Honestly.. dont use iphone to test things like this!    Apple device, especially battery devices choose power saving over performance at every opportunity, it's why their batteries last so long.  However it has the issue where it will sometimes cause slowness / reduced performance over WiFi as it attempts to save every drop of power.   I seen this thousands of times over the years, there is a reason WiFi Testers / Heat Mapper dont use Apple devices for this testing, they are not accurate.  Have in the back of your mind the expectation that Apple devices to be a tad slower vs Windows or Android

  @Philbert thanks for the help!