It has to do with the way Relays load and let clients sort through data. It isn't smart so if you have a situation where you want to view a historical feed clients can take a long time to grab that info. A good example would be something like if you make a new post, for the first few hours it will load fast. But as time since the post grows you may notice even if you're just viewing a history of your posts it might take a while to load.

I think this is the biggest problem with nostr at the moment. Relays need to be built smarter with better ways to organize, grab, and sort information.