Irrien wrote

my question is, should my devices automaticaly switch to the highest speed or do i have to look all my devices to see if they 2ghz or 5 ghz devices and then switch them all manually?

  @Irrien 

Like many dual band routers (i.e., broadcasting with same SSID on 2.4GHz and 5GHz) Deco mesh implements band steering. Band Steering means telling device which band to use. In Deco mesh, idle or lightly utilized devices steered to 2.4GHz, because that band drains mobile device battery less. When device starts utilizing WiFi network more heavily and for longer period of time, it will be steered back to 5Ghz. That might not happen for a quick speed test run.

My recommendations:

If you don't need 2.4GHz on Main Network, just turn it off.

If you do need 2.4GHz, with Deco M5 you have ability to configure preferred band per device, see Part 2 of the following document: How to select a preferred signal source on the Deco App

I would recommend making that configuration change only for devices that do need fast speed all the time: PC, laptop, tablet. Devices such as smartphones, Smart TV, IoT, WiFi printers - those don't really need to stay on 5GHz all the time, so you can leave this setting for them on its default "automatic" value.