tokio is great and it's pretty performant, but you pay an allocation for every future unless you do some complex organization of your futures.

Source: I worked on Deno, competed directly with Bun on HTTP performance (and won on some metrics).

Edit: and of course I typed future instead of task (aka "spawned future"). Thanks, child commenters below. Much of Deno was built on spawning futures that mapped to promises and doing it as fast as possible. I spent ages writing a future arena to optimize this stuff..