New Apple TV slow down my internet.

Hi. First step: be sure you've updated Apple TV to the latest tvOS 10.2. Second, use one of the built-in screensavers, not iCloud photos. When Apple TV is not actively streaming, it should not be using any network time at all. If it is, look through your settings for any feature that could be turned off.

The Wifi its really near to the Apple Tv but its very hard to connect it with ethernet cable (how important and helpful its to make a big change and connect the Apple TV directly to the router?)

As a hardwired connection is the best way tell me what is so hard about plugging it in?

I doubt your Apple TV is downloading that much data and suspect you are experiencing interference on your network whilst using the Apple TV and it is this that is slowing your network.

Oh and the reason you can't use Kodi is because Apple won't encourage you to steal.

You could try investing in some mains powerline network adapters which worked extremely well for me for many years and use the mins wiring to transmit data from one pint to the other (1 at router 1 near AppleTV) - I've subsequently had ethernet cable laid in many rooms and it's better still but not the sort of thing you can do casually and I understand you might be concerned by cable crossing the room being a trip hazard.

I have the exact same issue. When Apple TV is off I get my 15/5 download and upload speed. As soon as I connect, I go down to 1/1... tried with both my MacBook and iMac and the same thing happens. Disconnect Apple TV and I am back to normal. This makes no sense.

That would be because you don't get the interference when you turn off the Apple TV.

Do I need to buy an airport connection

No, not at all, if you are experiencing interference you are quite likely to suffer it with whatever equipment you use.

Check the cable you use to connect it to the network (if you connect with a cable)