This can be expected when there is a large number of items in the Gallery.
There is an option which will help:
"Only show media added within the last X days (Media Home)"
If you have a large gallery, your media home page may load slowly. This option will mitigate that to only show media on the media home page from the last X days. Older media is still discoverable via albums, categories and search. Set to 0 for unlimited.
If you change this value to something like "365" or "730" you should find the media index loads a lot faster. If you choose to only show 365 day's worth of media items in the index, it does not affect finding older images in albums or categories or via search.
I've known some busy galleries to actually only show 1-4 weeks of images on the media index. It keeps the index page less daunting and makes it more relevant, without hindering discoverability of older stuff.