My Xbox One is running horribly...

You could also check Xbox Live Status on this site. Xbox Live services do go down here and there from time to time.

My suggestion would be to do a hard reset like this. Hold the power button on the console for 10 seconds or until the power shuts down completely. Unplug the power cord from the back of the console for 5 minutes. Plug the power cord back in and restart
the console. Once it is back up and operational test for results. If that doesn't work and your internet connection is good then as suggested it may be time for a factory reset.

try doing a hard reset by holding the power button until it turns off, if thats doesn't work take Mr B's advice.

Make sure you're not in the Preview Program, hard reset, check to see if your console has the latest Update (they come out pretty much one update every month). Reset your internet modem too. Ask your ISP if they can remotely check your internet connection.
Spark NZ and Vodafone NZ can remotely check your internet connection. Sometimes when your ISP does this they may detect issues. Which country do you live in? Maybe lots of people in your suburb use cable internet. If they do, that can affect your internet
speeds since it's "Cable" internet. If it fails, contact Xbox Support. Maybe your console's hardware is playing up. Xbox 1 has an EIGHT CORE processor aka AMD Jaguar 8 core APU, so the hardware should be able to multi-task easily. The more processing cores
a device has the more easier it should be at multi-tasking. Try resetting your console to manufacturer default settings (how it was like when you first got it).

might try changing your DNS servers, while your connection to the internet maybe good, your cable company's DNS servers may not be sending your traffic through the best route. try using Google DNS or OpenDNS.

may also be a LAN issue, maybe a bad cable? maybe the network interface on the X1 is going bad? maybe try connecting it to the WiFi to see if that makes a difference.