Can I turn off Wi-Fi on the extreme, like you can on express ie ‘join a wirelesss network’.
No, since there is no setting or option on an AirPort Extreme to "Join a wireless network". The AirPort Express is the only AirPort device that can "Join a wireless network" and provide an Ethernet signal at the same time.
In addition, the AirPort Extreme will not be able to "Extend a wireless network" unless it will be connecting to a Wi-Fi signal that is provided by another Apple AirPort router. Or in other words, the AirPort Extreme cannot connect to a 3rd party router using a Wi-Fi signal.......it must connect to a 3rd party router using a permanent wired Ethernet cable connection.
My Wi-Fi speeds have dropped and I’m wondering if it’s because of too many Wi-Fi signals in one house?
The Wi-Fi network will slow down as the number of Wi-Fi devices connect to the network increases. So, if you have a 50 Mbps Internet Connection with your provider and things are slowing down on the network, you will need to look at a 100-200+ Internet connection plan.