Composer perf: preload components to avoid parsing javascript on “reply”

app/src/global/mailspring-component-kit.es6

@@ -5,6 +5,24 @@
 // `require` files the first time they're called.
 module.exports = exports = {};
 
+// Because requiring files the first time they're used hurts performance, we
+// automatically load components slowly in the background using idle cycles.
+setTimeout(() => {
+  const remaining = Object.keys(module.exports);
+  const fn = deadline => {
+    let key = null;
+    let bogus = 0; // eslint-disable-line
+    while ((key = remaining.pop())) {
+      bogus += module.exports[key] ? 1 : 0;
+      if (deadline.timeRemaining() <= 0) {
+        window.requestIdleCallback(fn, { timeout: 5000 });
+        return;
+      }
+    }
+  };
+  window.requestIdleCallback(fn, { timeout: 5000 });
+}, 500);
+
 const resolveExport = requireValue => {
   return requireValue.default || requireValue;
 };