Vapor generally kicks ass in benchmarks FWIW, and it's using this now so clearly Apple is beginning to pay attention to stuff around server side concurrency: https://github.com/apple/swift-nio
Although as I mentioned in another comment, if you're looking for something more mature, Kotlin is a great option. Vert.x has the widest database support and is lean and fast, so this is probably what you're looking for. But if just Mongo/Cassandra/Redis is acceptable, Spring Boot w/ Reactive Web looks pretty cool, too.
Although as I mentioned in another comment, if you're looking for something more mature, Kotlin is a great option. Vert.x has the widest database support and is lean and fast, so this is probably what you're looking for. But if just Mongo/Cassandra/Redis is acceptable, Spring Boot w/ Reactive Web looks pretty cool, too.