Yes indeed this might be a problem – but not just with the latest version 1.99, this would also affect older versions as well.
Lightbox with PhotoSwipe tries to get the image sizes by reading the image meta data based on the URL – and this happens in your server, not in the browser. This is required, since PhotoSwipe needs to now the width and height of an image before the lightbox is created.
It may be that CDN does not allow access from your server or that the CDN request times out since the regional cache of the CDN does not have the image yet.
Can you tell me what plugin you used to get your images hosted on a CDN? Or did you turn off the CDN integration completely? Maybe I will add an option in the backend settings where one can enter the URL of the CDN, so it will get replaced by the real URL on the server before the plugin tries to load the image data.
Oh – and just for the records: this whole process is only done mostly once a day per image and not with every page view. So no worries about increasing the traffic by using my plugn ;-).