@Decorer247 

Given the link speed (961/1021) showed by Windows, I believe obtaining 400Mbps is a good result.  

When you obtain 400Mbps via "openspeedtest-server", are both your devices (macbook and pc) connected by wifi to the same Deco?

25Mbps should be sufficient for 4K streaming.  What is your setup when two devices are streaming, how are the three devices (2 clients and 1 server) connected to the Decos? 

The Deco are using a common channel for each band.  Is there a possibility that the wireless traffic of one Deco interfers with the wireless traffic of another Deco? 

I assume that the Deco app shows the Ethernet backhaul. 

  @yves_b thanks for the reply.

you said 400mbps is enough but why not full 1000mbps as per link speed. what is eating up my speed on LAN  side? it should not happen.  could you please explain ?

here are answers to few of your question-

When you obtain 400Mbps via "openspeedtest-server", are both your devices (macbook and pc) connected by wifi to the same Deco?

yes they connect to same deco.

25Mbps should be sufficient for 4K streaming.  What is your setup when two devices are streaming, how are the three devices (2 clients and 1 server) connected to the Decos? 

all are connected via wifi 5ghz and both streaming devices are apple tv.          one apple tv connects to 1st deco and other apple tv connectes to second deco and the 1 server connects to main deco.

The Deco are using a common channel for each band.  Is there a possibility that the wireless traffic of one Deco interfers with the wireless traffic of another Deco? 

Both the decos are separated by two rooms and via the deco app i have manually selected that what device choose which deco.

I assume that the Deco app shows the Ethernet backhaul. 

yes all decos are connected via ethernet.

  @Decorer247

 I see that a deco X20 has those theoretical maxima:

5 GHz: 1201 Mbps (802.11ax)
2.4 GHz: 574 Mbps (802.11ax)

Windows shows a (theoretical) maximum of (961/1021) on 5Ghz.

Those maxima are achieved in labs, with optimal conditions.

In your test, when you reach 400 mbps between two devices connected to the same Deco, you consume 800 mbps, 66% of the theoretical maximum of the Deco.

The wifi is a radio signal, it is possible that a transmission from one device interferes with a transmission from another device, even if the recipient is different.

I still do not understand why you have lagging issues with the streaming.

  @yves_b thankyou so much for this great explanation. 

  @yves_b Please explain me one thing,

you said above that I am getting "In your test, when you reach 400 mbps between two devices connected to the same Deco, you consume 800 mbps, 66% of the theoretical maximum of the Deco."   so when I purchased the decos x20 when it said it could do 1000mbps as LAN speeds as shown in link speed in windows, I thought It meant I would be getting 1000mbps up and down in local network but you told me that I am getting 800mbps but am confused with just only 400mbps up and down speed.

thankyou

thank you so much , I got your point, very well explained.

so for met to get 1000mbps up and down, Shall I buy Deco x60 or which one would you recommend for that sorts of need.

also does does streaming on LAN / local network 4k movie affects internet speed or nothing happen to internet speed ?

thanks