The CPU usage is caused by creating thumbnails. During upload, the first thumbnail is created because you see a preview of the uploaded image. While opening the event page, more thumbnails are created. Having a watermark on or not does not make much of a difference.
How large are your images? This has a huge impact on CPU usage. 30MP images are more expensive than 2MP images. There is a tiny trick you can try: https://www.svenbluege.de/joomla-event-gallery/event-gallery-manual/faq-original-images
Upload smaller images and the original files separately. This will allow fast thumbnail creation but you can sell the original image.
If you want to calculate the thumbnail on your local machine, you can try this too. Please have a look at the /cache/com_eventgallery_images folder.
Valid and necessary thumbnail sizes are: 48, 104, 160, 288, 320, 400, 512, 640, 720, 800, 1024, 1280, 1440, 1600
This describes always the larger edge or an image. The images are prefixed with crop or nocrop. Crop means a square sized image. If you miss to create a thumbnail or used an invalid file name, Event Gallery will still create the thumbnail itself.