SoggyHippo

OP, if you can run MSI Afterburner with the onscreen display while playing games with it set to show your CPU usage %, Temps, and clock speed. As well as GPU usage %, Temps, clock speed. Also RAM usage might help as well.

What graphical settings in game are you using?
 
Is it just a short, momentary drop in frames and then recovers back to 140FPS, or does the FPS gradually get lower and lower the longer you play?
When it does drop, what does it drop to?
Do the drops in FPS coincide with action happening on screen, like intense fights with other players, or entering a large city?
 
Is that 2x8GB sticks or a single 16GB stick?

PS. Don't feed the April Fool trolls.

By default vsync is off so it doesn't matter you have a 60hz monitor.  Your CPU usage should not hit 100 percent, perhaps 60 percent but not 100 percent.  Something is fishy.  I recommend running DDU "Display Driver Uninstaller" in safe mode, clean and restart and back into normal mode install the latest WHQL drivers and let us know how Fortnite runs.  By default the drivers are set to application controlled for vsync.  Make sure you don't have vsync on in Fortnite.  Let us know what happens.  Good luck!

Try to turn details down one notch and maybe same with AA