When you pick "federated" you'll see all posts, independent from your instance. But that's pretty much impossible because unless you have Tusky the posts will be too fast.
So yeah, to be able to read anything you have to just read the posts on your instance, meaning it does matter which one you pick.
It's not even the speed. It's also the content. All from all instances seems cool until you can't read 80% of it because it's in another language. I'm glad those instances exist but when I have to scroll through 5 posts to see one I can read, and it's some low-effort post (on average) the experience is not a good one
Even after all that work, you've gotten through 3 seconds of the world's timeline
Local is a good one, if your instance is lively enough. Then you can add people and see boosts and your world grows nicely
I find the "explore" page super useful for exploration! Spend a few minutes twice a day or so and the content quality is quite high, great place to branch out into boosts from. On my instance, I also do my best to curate the global feed to be somewhat useful (though things to creep in at the edges, I need to do some culling now for some Misskey bridges that are pretty loud in that feed).
added in March of last year, and not sure about third party apps since I use the official interface. I find it very useful though, between that, hashtags, public lists of people to follow, and boosts, I set my feed up in about 6 hours and it's been on minimal-maintenance-mode since and serving me fine. depends on how much the niche communities you like adopt the Fediverse too, YMMV!