It is important to try out your sessions before you do your actual session with live participants.
You want to make sure that the session displays the various slides intended. But, especially if you have many slides (with images that nowadays are often quite large), it is important that you go through the entire session at least once before you go live.
This will ensure the images are cached by the browser and QandR and subsequent loads will be a lot faster.
Start the session in the dashboard and log in with your phone (using the QR code or the participant link). That way, you immediately experience what participants see, and QandR and your internet browser get acquainted with your slides.