from https://github.com/Microsoft/visualfsharp/pull/930#issuecomment-179602560

930.fs

type EnumA = A0 = 0 | A1 = 1 | A2 = 2 | A3 = 3 | A4 = 4 | A5 = 5 | A6 = 0x7
let getAnEnum i = match i &&& 7 with 0 -> EnumA.A0 | 1 -> EnumA.A1 | 2 -> EnumA.A2 | 3 -> EnumA.A3 | 4 -> EnumA.A4 | 5 -> EnumA.A5 | _ -> EnumA.A6

type R = {
    GroupByKey : EnumA
    OtherData : float
}

let data = [|
    let r = System.Random 42
    for i = 1 to 10000000 do
        yield { GroupByKey = getAnEnum (r.Next()); OtherData = 42. }
|]

let total_time = System.Diagnostics.Stopwatch.StartNew ()
for i = 1 to 10 do
    let sw = System.Diagnostics.Stopwatch.StartNew ()
    let grouped =
        data
        |> Array.groupBy (fun d -> d.GroupByKey)
    printf " iteration=%dms [" sw.ElapsedMilliseconds
    grouped |> Array.iter (fun (key,items) -> printf "%A=%d;" key items.Length)
    printfn "]"
printfn "total_time=%dms" total_time.ElapsedMilliseconds