While streaming a 450 MB / 6.6 Mio records file from Zip format on disk into memory as a sequence works with adequate performance, but turning that sequence into a map via `Map.ofSeq` seems to be practically impossible:

```
    // within a let binding ...
    getPart fileName xPath partUri
    |> fun x -> printfn " getPart finished, parsing ..." ; x |> Seq.map (fun x -> 
        let test name = 
            let x' = (xd x).Root.Descendants() |> Seq.filter (fun x'' -> x''
            // do some System.Xml.Linq stuff here)
    |> fun x -> printfn " Parsing finished,  reorg seq ..." ; x |> Seq.map (fun x -> x.Reference, x )
    |> fun x -> printfn " Reorg finished,  building cell map...   ### see this ###  " ; x |> Map.ofSeq

do printfn "%s" " ### never see this ### done, now building keys..."

```

Is this a known issue or am I making a mistake?