Map vs Dictionary.fs

module MapVsDictionary

open FSharpx.TimeMeasurement
open System.Collections.Generic

let keys = [1..500000]

compareTwoRuntimes 
    10
    "Map"
    (fun() -> keys |> List.fold (fun m x -> m |> Map.add x x) Map.empty)
    "Dictionary"
    (fun() -> keys |> List.fold (fun (d: Dictionary<_,_>) x -> d.Add(x, x); d) (Dictionary<_,_>()))

// results:
// Map 994.7ms
// Dictionary 39.6ms
//   Ratio:  25.11868687

let m = keys |> Seq.map (fun x -> x, x) |> Map.ofSeq
let d = Dictionary<_,_>()
keys |> Seq.iter (fun x -> d.Add(x, x))

compareTwoRuntimes 
    10
    "Map"
    (fun() -> keys |> List.iter (fun x -> m |> Map.find x |> ignore))
    "Dictionary"
    (fun() -> keys |> List.iter (fun x -> d.Item(x) |> ignore))

// results:
// Map 187.9ms
// Dictionary 12.0ms
//   Ratio:  15.65833333