Elves are incredibly spiritual and lack the drive that humans have. They take up decades long hobbies that are just for the sake of that thing and then just do nothing but obsess over it like they have turbo autism. You might get a few elves who have their wild years but an elf can choose to get pregnant or not. low birth rates are for the same reason humans don't have children. Imagine you had a kid but they were a "child" well into their 60s. Its not that they aren't capable its that elves rely on complex social structures and norms. where as a human goes though his whole life in 60 it takes an elf almost 300. Why? They are just floating through time, not a care in the world. Humans have that fear of death and mortality that elves lack. Most elves spend a good solid 20 as children, another 20 as teens, and another as young adults. They party and fool around for nearly 60 years while a human has had almost 4 generations and helped build a legacy that ensures his family's success. Elves have it so easy that they rarely feel necessity. Necessity is the mother of invention so it takes them a century to deal with something a human would want done in 50. Usually they see things as "If we just ignore this problem it goes away." They don't usually have to worry about most issues.
Elves really on average aren't that much better than people. Elves are slight and slender. Much weaker physicality and a lot of that "patience" is actually just blind foolish ignorance or child like wonder. Elves are VERY emotional until they get to about 80 or so and feel very strong emotional bonds to family and duty but this is also a bit of a curse. Being fickle and fey they act like spoiled teenagers for nearly a century and then finally realize if they calmed their tits for a decade they could actually master a skill.
Elves like Legolas? Those are the weird autistic elves that get obsessed about something and just won't put it down. Basically a /r9k/ elf obsessed with bows.