Here’s a useful topic which explains Heartbeat / RenderStepped better than I could, I recommend reading it.
This might lag as it’s checking distances very frequently. Try some benchmarking tests to really make your decision though - try with as many ghosts as you can until it lags, and ask a friend with different hardware specifications to try it too.
You could also try detecting if a ghost is inside of an invisible part determining the range of the torch, which may be less laggy.