Does Ethernet Affect Wifi?

To summarize, regardless of what you want to do online, an ethernet connection will always be faster and more reliable than wifi and it won't affect wifi speed.

Does an Ethernet cable slow down Wi-Fi for others?

The correct answer is that using an ethernet cable instead of using wifi won't slow down the rest of the devices connected to your home network. In fact, the other devices should actually get faster, as there's one less devices using the wireless network.

Should I turn Wi-Fi off when using Ethernet?

Wi-Fi doesn't need to be turned off when using Ethernet, but turning it off will ensure that network traffic isn't accidentally sent over Wi-Fi instead of Ethernet. It can also provide more security as there will be fewer routes into the device.