Seq.windowed があるなら buffered があっても良いじゃない。

buffered.fs

let buffered<'a> bufferSize (source : seq<'a>) =
   seq {
      use enumerator = source.GetEnumerator ()
      let incomplete = ref true
      while !incomplete do
         let buffer =
               seq {
                  let request = ref bufferSize
                  while !request > 0 && enumerator.MoveNext () do
                     yield enumerator.Current
                     decr request
               } |> Seq.toArray
         if buffer.Length > 0
         then
            yield buffer
         else
            incomplete := false
   }

(*
igeta 氏の https://github.com/igeta/FsugJp.FSharp.Collections を使う場合以下が同等
toArray が冗長に見えてくるかも
*) 
let buffered bufferSize =
   Seq.zip (Seq.initInfinite id)
   >> chunk (fun (i, _) -> i % bufferSize = 0)
   >> Seq.map (Seq.map snd >> Seq.toArray)