F# List optimisation

From an unordered list of int, I want to have the smallest difference between two elements. I have a code that is working but way to slow. Can anyone sugest some change to improve the performance? Please explain why you did the change and what will be the performance gain.

let allInt = [ 5; 8; 9 ]

let sortedList = allInt |> List.sort;

let differenceList = [ for a in 0 .. N-2 do yield sortedList.Item a - sortedList.Item a + 1 ]

printfn "%i" (List.min differenceList) // print 1 (because 9-8 smallest difference)

I think I'm doing to much list creation or iteration but I don't know how to write it differently in F#...yet.

Edit: I'm testing this code on list with 100 000 items or more.

Edit 2: I believe that if I can calculte the difference and have the min in one go it should improve the perf a lot, but I don't know how to do that, anay idea?

Thanks in advance