For the former, this usually happens when Epic is deploying a major update or patch, and the latter can come about when an enormous amount of players are online at one time. If Fortnite is having some intense lag issues, make sure your game is properly installed.
From time to time, Fortnite: While Fortnite automatically connects to your closest matchmaking region, you may want to experiment with the settings and see if some regions are more reliable to connect to than others. In other words, port forwarding optimizes your internet connection by telling your computer and router how to talk to each other. If you find yourself equidistant to two different matchmaking regions, try switching between them and seeing which one provides the most stable connection. Port forwarding is an advanced technique that lets users specifically tell a computer how to send network packets through a local area network and onto the internet, streamlining communication for users on the client side of things. Quality of Service instructions vary from router to router, but logging into your router settings from your web browser should be all that you need.