In the solution of the Pangram exercise:

let totalAlphabetCount = 1 + int ('Z' - 'A')

let isPangram (input: string): bool =
    input
    |> Seq.filter System.Char.IsAsciiLetter
    |> Seq.map System.Char.ToUpper
    |> Seq.distinct
    |> Seq.truncate totalAlphabetCount
    |> Seq.length = totalAlphabetCount

Does Seq.truncate totalAlphabetCount improve performance in huge input?
For example, we have a 1Gb string with all ASCII letters in the first hundred characters.