Use the native loading indicator with turbolinks

loader.js

var frame;
var frameSrc = "/sleep" // some local endpoint that just `sleep 2` or something

function addFrame() {
  frame = document.createElement("iframe")
  frame.style.display = "none"
  document.head.appendChild(frame)
  frame.src = frameSrc
}

function removeFrame() { 
  try {
    document.head.removeChild(frame)
  } catch (e) {
  } 
  frame = null;
}

$(document).on("page:before-change", addFrame)
$(document).on("page:load", removeFrame)