Timing on Seq.toArray with ICollection<'T> input

Program.fs

// Learn more about F# at http://fsharp.net
// See the 'F# Tutorial' project for more help.

open System.Diagnostics

[<EntryPoint>]
let main argv = 
    let sequ = seq [ 1 .. 10000000 ]    
    let cach = ResizeArray<_>(sequ)

    let doTimings s =
        let sw = Stopwatch.StartNew()
        let arraySecond = cach |> Seq.toArray
        sw.Stop()
        float(arraySecond.Length), float(sw.ElapsedTicks)

    printfn "pre-running for JIT"
    let arrayFirst = cach |> Seq.toArray
    let init = doTimings cach

    let iterations = [1..10]

    printfn "starting iterations"

    let results = iterations |> List.map (fun _ -> doTimings cach)

    let averageLength = results |> List.averageBy (fun l -> fst(l))
    let averageTime = results |> List.averageBy (fun l -> snd(l))

    printfn "Length %d=%f in %f ticks" arrayFirst.Length averageLength averageTime

    0 // return an integer exit code

Thanks for providing perf test code with your PR, makes it very easy to confirm behavior change.