Hello,
Read this post https://www.wpbeginner.com/beginners-guide/speed-wordpress-save-images-optimized-web/ it explains how to save images and how to optimize them.
If I check the product I don’t see problem with the gallery anymore https://gyazo.com/3c9ad19034555687b4555a9be858db9d
But images are so big that they loading too slow.
Regards