Today I booted up my UEFN editor and I can’t move around in the viewport. I’ve recently added a Niagara VFX and installed Perforce Helix Core and Admin (which I have subsequently uninstalled) but I don’t believe I’ve done anything else of consequence in the last 24 hours.
When I launch the game on Fortnite, it is also very laggy.
First, go to Edit → Editor Preferences → Performance → Enable 'Show Frame Rate and Memory". This will help you put some numbers to what you’re seeing.
For a moment I thought you were talking about the default fireworks VFX that starts going off when you place down the ‘VFX Spawner’ device. But it sounds like you’re creating some custom VFX.
If you go through the tutorial again, but this time paying attention to which step causes your FPS to drop, you might get a better idea of which aspect of this VFX you’d need to tune. I’d also try taking your learnings from that tutorial and creating a simple fireworks effect, and see how that performs.