Unhappy Lag when opening aps (HTC One)

Hello,

My phone is barely 3 weeks old. However I have realized that lately my phone lags when opening specific apps. Especially Youtube, Facebook, Facebook Messenger and Outlook.com

Anytime I launch the apps, there are a few seconds of lag before anything happens... It is become really frustrating, and I can't believe that my new phone could be this slow already.

I even reformatted the phone, but that didn't help...

What can I do?

Thanks