"With shadows on, I get like 19 FPS in the base. Shadows off, I get 60."
Settings -> Graphics -> Advanced-> change "Graphics RHI" from DirectX11 to 12 or Vulkan. Engine just can't handle shadows in DX11 mode for some weird reason. TBH 12 probably ought to be made the default; you'd kill like 90% of framerate complaints, and it's not like any GPU old enough to top out at 11 is gonna have the horsepower to boot this thing anyway.