Raspberry Pi 3b v1.2 when connected to WiFi it nearly "kills" the network

[Hobb7T]

Greetings fellow programers and enthusiasts!

Lately i installed Ubuntu MATE on my RasPi 3b v1.2 and i faced many problems with the WiFi. Most of the times the WiFi didn't worked at all, or some times i was connected but couldn't ping to anything, so i did a fresh install back to Stretch. Now im still facing problems while im connected to the WiFi.

When im connecting the Raspberry on the router, all the devices on the network seems to extremely slow down (when the RasPi is connected i get 2.5Mb from speed test on my Macbook, while its off i get 19.5Mb)
When i try to apt-get update it just failing as the device is not capable to download the updates. Im using the same power adapter i used before (as i read on many forums that the power supply can make problems tho)
Im using an HDMI as before connecting on my screen, using the GUI. (I mean i did as i did when i first started using the product, but now im facing problems)


Any thoughts?

[epoch1970]

Are you using static IP addresses? Maybe there is a conflict. Quite likely.

3B operates over the 2.4 GHz frequency band only, not 5GHz as other machines might. Normally 5GHz capable clients will not be switched to 2.4 unless the router is a (cheap) dual band, single radio design. If clients are switched to 2.4, check your router settings for unusual parameters. Somewhat likely.

The description is also somewhat consistent with a network loop, in case the pi is also connected via Ethernet at the same time, try disconnecting the cable and see if anything changes. If so, examine the network hardware and topology. Very unlikely.