Firebase Performance Monitoring issue

Why does the following JavaScript work on an android-device:

firebase.initializeApp(firebaseConfig);
ana= firebase.analytics();
ana.logEvent("BlaBla");

But the following does not work:

perf = firebase.performance();
//alert(perf.dataCollectionEnabled);
var t = perf.trace('InitializeNew');
t.start();
setTimeout(function(){ t.stop(); }, 3000);

Somehow my custom-trace does not appear in Firebase Performance, only when I use the PWA (Web) version.