Faster Array.partition for F#

partition.fs

let arrayPartition f (array: _[]) = 
    checkNonNull array
    
    //Hold both arrays in one of exact length
    let res = Array.zeroCreate array.Length        
    let mutable upCount = 0
    let mutable downCount = array.Length-1
    for i = 0 to array.Length-1 do
        let x = array.[i]
        if f x then 
            res.[upCount] <- x
            upCount <- upCount + 1
        else
            res.[downCount] <- x
            downCount <- downCount - 1
                
    //First array is just a subset
    let res1 = Array.sub res 0 upCount

    //Second array needs to be reversed
    let res2 = Array.zeroCreate (array.Length - upCount)
    downCount <- array.Length-1
    for i = 0 to res2.Length-1 do
        res2.[i] <- res.[downCount]        
        downCount <- downCount - 1

    res1,res2