"terrible" here means within the ballark of Express and usually lower than Phoenix (the Elixir web framework), far slower than Go, and than most Java framework. The source is the TechEmpower framework benchmarks, with the caveat that Vapor isn't in all the categories, and the usual benchmarks caveat.

For why it's not performant, I don't know precisely. Part of it may be because people don't use it a lot on the server side so it's not as optimized as the others. From what I've heard, Swift was also optimized for low memory usage rather than raw speed. Outside of that, I don't know. I would expect it to be around Go.