Async performance bug

async-perf-bug.fsx

let run x = Async.RunSynchronously x

let runParallel (workflow : Async<'T>) =
    [| for i in 1 .. 100 -> workflow |] 
    |> Async.Parallel
    |> run

let a = async { return 42 }
let b = async { return run (async { return 42 })}

#time "on"

run a // Real: 00:00:00.000, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0
run b // Real: 00:00:00.000, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0

runParallel a // Real: 00:00:00.031, CPU: 00:00:00.015, GC gen0: 0, gen1: 0, gen2: 0
runParallel b // Real: 00:01:24.086, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0

// UPDATE: seeing the same behaviour when using inside tasks

open System.Threading.Tasks

// Real: 00:01:21.493, CPU: 00:00:00.062, GC gen0: 2, gen1: 2, gen2: 1
[|for i in 1 .. 100 -> Task.Factory.StartNew(fun () -> run(async { return i})) :> Task |] 
|> Task.WaitAll