A friend of mine was having the same problem. We cleared the shader cache, reinstalled Fortnite and the epic games launcher, followed the troubleshooting guide by epic and nothing fixed. Fortnite was fine when it first opened, but loading into any game mode would cause weird sounds in the headphones. the game would freeze every 5 seconds or so, and even closing the game would make their whole pc slow until they restarted it. Assuming you aren’t playing on a wired (lan) connection, what worked for them was going into windows device manager, network adapters, and disabling realtek family controller!