(1) Cloud Haskell is primarily designed for execution on distributed memory architectures where the messages pass over a network. So expect that the it would show high communication latency in the first PingPing benchmark.
(2) GHC Haskell processes are lightweight, i.e. maintained in the GHC Runtime Environment, and elsewhere in this discussion called fibres. So expect that it would have fast process creation and be able to sustain a large maximum number of processes (second benchmark).
(3) It's hard to speculate on throughput (third benchmark). The high communication latency may be hidden in a steady state by having multiple requests in the network. Haskell execution time is slower than many languages, but is probably comparable with Erlang and Scala.
There were many languages that we would like to have considered. We do expect the performance of Elixir to be pretty similar to Erlang as it runs on the same VM (BEAM).
With regards the "alternatives considered", Section 2 outlines key characteristics of server languages, and Table 1 analyses a selection of languages against these characteristics.
We welcome others to report results for other languages using the benchmarks.