After many problems, it's been difficult to find which settings with Akka and Spray have an impact on performance. This is an attempt to put all the settings that seem like they matter, in one place.

comprehensive_spray.io.conf

  akka.actor{
    creation-timeout = 20s
    default-dispatcher {
      throughput = 20
      executor = "fork-join-executor"
      fork-join-executor {
        parallelism-min = 16
        parallelism-factor = 2.0
        parallelism-max = 16
      }
    }
  }

 server {
    server-header = spray-can/${spray.version}
    ssl-encryption = off
    pipelining-limit = disabled
    idle-timeout = 60 s
    request-timeout = 20 s
    timeout-timeout = 25 s
    chunkhandler-registration-timeout = 1 s
    timeout-handler = ""
    reaping-cycle = infinite
    stats-support = off
    remote-address-header = off
    raw-request-uri-header = off
    transparent-head-requests = on
    chunkless-streaming = off
    verbose-error-messages = off
    request-chunk-aggregation-limit = 0
    response-header-size-hint = 512
    max-encryption-chunk-size = 1m
    bind-timeout = 10s
    unbind-timeout = 10s
    registration-timeout = 10s
    parsing-error-abort-timeout = 2s
    default-host-header = ""
    automatic-back-pressure-handling = on
    ssl-tracing = off
    parsing = ${spray.can.parsing}
  }

 	host-connector {
    max-connections = 32
    max-retries = 20
    max-redirects = 0
    pipelining = off
    idle-timeout = 30 s
    client = ${spray.can.client}
  }

  parsing {
    max-uri-length             = 2k
    max-response-reason-length = 64
    max-header-name-length     = 64
    max-header-value-length    = 8k
    max-header-count           = 64
    max-content-length         = 8m
    max-chunk-ext-length       = 256
    max-chunk-size             = 1m
    uri-parsing-mode = strict
    illegal-header-warnings = on
    header-cache {
      default = 12
      Content-MD5 = 0
      Date = 0
      If-Match = 0
      If-Modified-Since = 0
      If-None-Match = 0
      If-Range = 0
      If-Unmodified-Since = 0
      User-Agent = 32
    }
    incoming-auto-chunking-threshold-size = infinite
    ssl-session-info-header = off
  }

  manager-dispatcher = "akka.actor.default-dispatcher"
  settings-group-dispatcher = "akka.actor.default-dispatcher"
  host-connector-dispatcher = "akka.actor.default-dispatcher"
  listener-dispatcher = "akka.actor.default-dispatcher"
  connection-dispatcher = "akka.actor.default-dispatcher" 

  spray.io {
  tcp {
    keep-alive = 1
    no-delay = 1
  }
}