As with most online games, even in Fortnite LAG is a naturally occuring phenomenon that plagues all gamers. But lag is not always due to network issues, sometimes lag has to do with the hardware of your rig and/or the software. So before you can solve Fortnite lag, you need to diagnose whats causing the high ping in the first place.

Visit the official Fortnite website to check the recommended system requirements, and make sure that you meet both hardware and software requirements.

Make sure you are connected via ethernet cable and not wifi. I know this is a small thing and shouldn't matter in this day and age, but, trust me it helps with ping.

You can try to change your matchmaking region so you get better ping.

If this doesn't help, here you can read up on more workarounds for Fortnite lag including network related issues.