Why Does Fortnite Lag My Whole PC Every Few Seconds?

I've been experiencing a frustrating issue with Fortnite that's affecting my entire PC performance. Initially, everything runs fine in the lobby, but once I ready up and enter a match—whether in Battle Royale or Creative—the game starts downloading shaders. Almost immediately, my whole system begins to lag, freezing every couple of seconds. My FPS drops to around 20, and the screen freezes for about half a second before jumping back to normal, repeating this cycle continuously. This problem doesn't just impact Fortnite; it also interferes with other applications, causing audio cuts, mouse stuttering, and even freezing browsers like YouTube if the game is still running. The freezing persists in the lobby after I leave the match until I close the game entirely. I've already performed a clean install of Windows, reinstalled GPU drivers using DDU, updated my BIOS, and ensured that no unnecessary programs are running. My specs include a Ryzen 7 9800X3D and an RTX 4070 Ti SUPER, and I've only had this setup for five months. I'm out of ideas and really need some help to resolve this issue.