Question about ASUS RT-AC68U, AC1900 AiMesh

There have been no LAN hubs (double plugs) since the turn of the century, as switches are now very cheap.
This model is a WLAN router with a built-in LAN switch.

The part about "shuts down the speed for the whole network" is a common fairy tale. :)
A wifi router usually has LAN ports and thus also becomes a LAN switch (LAN HUBs are "obsolete" and should not really play any role in the private environment, as they work very inefficiently and can do less than switches). If you are interested in more detail, you can read about it on Wikipedia. But in general, nobody needs LAN hubs any more.

This is a WLAN router.