@tsak 

I have three suggestions.

1. Check that each Satellite Deco uses Ethernet for backhaul. Just because you connected Satellite Deco with Ethernet cable, it is not guaranteed it will. In Deco app, open each Satellite Deco page and check what it reports under "Connected to:". If Satellite Deco uses Ethernet, this is what you should see:

2. It is possible that your Main Deco became a bottleneck, if you wired your Decos like this:

If you did, you should rewire them as following:

3. Your ISP link is gigabit down and 100Mbps up. Video calls is symmetrical traffic, meaning upload could be the same as download. Which means, when many clients are making video calls your ISP link is no different from 100Mbps down and 100Mbps up. Is it enough? That might not be, especially if video calls are made with 4K or HD quality. 

The solution could be do reconfigure video calls to run with less than HD video quality, and/or search for an ISP that provides symmetrical gigabit link (or at least the link with faster upload than 100Mbps). 

  @Alexandre. 

Thanks for the reply. The Network setup is the second one you've posted, with all 4 nodes connected after the switch.
100Mbps is not optimal, but it should be enough for 15-20 video calls even on HD.

Would it make any difference if I disabled the mesh feature for the laptops, since they're mostly stationary?

  @tsak 

Have you checked if Satellite Deco report using Ethernet for connection?

You can disable mesh for stationary devices, but as long as they are connected to closest Deco unit that won't matter much.

Also, consider turning off WiFi 2.4GHz band on Deco Main Network, just to see if that will improve video call quality. 

  @Alexandre. 
All nodes are reporting Ethernet. I've also disabled 2.4GHz.
The connection overall is very good. The issue happens only when clients are doing many video calls simultaneously.
I can also enable Qos or priority for the macbooks on the floor.

  @tsak 

You have optimal Deco mesh configuration. I still suspect that either there is something else that adds to Internet link utilization, impacting overall performance, or perhaps your ISP router can't handle total traffic. 

There is another test you can do to check if Deco units are under stress. This can use Main Deco Web interface. If you open Main Deco IP address in browser, you should see Deco mesh status page, and if you scroll to the bottom of it you will see Main Deco CPU load and memory utilization. It is updated in (almost) real time:

This will be the test. Figure which Deco has most simultaneous voice calls going through it. If it happens to be not the Main Deco, swap that Deco with Main Deco, that won't make the difference in AP mode.

Then, wait till peak time for voice calls, login to Main Deco Web interface and check its current CPU and memory utilization. Give it few minutes to settle down and provide you with steady info.

If you don't see heavy CPU or memory utilization on Main Deco under busy time for voice calls, that should mean Deco unit handles them well and you may need to focus on ISP router and Internet link utilization.

There are no settings in Deco mesh running in AP mode to throttle or prioritize traffic. Such settings can impact user experience, but in AP mode they are disabled. If you are not hitting X20 hardware limitations (CPU/memory), the bottleneck must be somewhere else.

  @Alexandre. 
I did check the CPU load and memory metrics. The memory is always at 52-53% and the CPU varies from 50 to 70%, so I don't think it's a hardware issue. For the time being I will consider this as an upload speed bottleneck from my ISP. So thank you for you help. Let's hope there will be also some firmware update, that addresses the stress issue.