F# boxing equality

boxing.fs

open System.Collections.Generic

[<Struct; CustomComparison; CustomEquality>]
type MyVal  =
    val X : int
    new(x) = { X = x }

    override this.Equals other = 
        match other with
        | :? MyVal as y -> this.Equals y
        | _ -> false

    override this.GetHashCode() = this.X

    member this.CompareTo (other:MyVal) = this.X.CompareTo(other.X)
    member this.Equals (other:MyVal) = other.X = this.X
    with 
        interface System.IComparable<MyVal> with
            member this.CompareTo other = failwith "never comes to here"

        interface System.IEquatable<MyVal> with
            member this.Equals other =  other.X = this.X
            

let inline contains (e:'t) list1 =
    let comp = EqualityComparer<'t>.Default;
    let rec contains e xs1 =
        match xs1 with
        | [] -> false
        | h1::t1 -> comp.Equals(h1,e)|| contains e t1
    contains e list1

let count = 5000
let s = [for i in 0 .. count do
            yield MyVal(i)]

System.GC.Collect()
#time

for i in 0 .. count do
    contains (MyVal(4999)) s |> ignore
#time
printf "done"
// Real: 00:00:00.135, CPU: 00:00:00.135, GC gen0: 0, gen1: 0, gen2: 0