Hey Russ,
It looks like your projectile is doing a lot over network. A useful tool for determining load on network is Network Profiler. Run your game and use console command, “netprofile” on server, then fire projectile a bunch. Exit Play and find .nprof file for session and open it in standalone network profiler tool.
I did this for your game and compared it to a blank First Person Blueprint template project (with projectile set to Replicates and Replicate Movement). projectile on FP project has a much lower profile on network than one in your project.
My suggestion would be to remove as much of functionality as you can out of projectile class itself, and instead call functions in other classes or from other classes as appropriate. If you place projectile from FP template in your project and spawn that instead, I think you’ll notice lag ceases.
Hope that helps!