Nostr doesn't have any images on it, so it can't be slow to load them. Nostr only has notes.

Images, videos, and other media must be hosted somewhere else, like a Blossom servers or nostr.build, or anywhere else that will give you a URL to access the media after you upload it.

How fast images load will depend on many factors, including the bandwidth of your media host, how the Nostr app you are using fetches that media (for instance if it uses a proxy in-between), and your own internet connection, etc.