The indentation makes your snippet quite hard to read - the VB.NET syntax for inline delegates is fairly bulky (at least compared to C#), I'd give it more air; being able to read the code is step one.
Dim lst = (From t In DocElet.ChildNodes Select ID = t.item("ID").outerxml).Distinct() .ToList() Parallel.For(0, lst.Count, Sub(i) Dim P As XmlElement = GetElement(lst(i)) Dim ls = (From t In DocElet.ChildNodes Where t.item("ID").innerText = P.InnerText Select t) Parallel.ForEach(ls, Sub(D) Dim verif_date As String = D.Item("DAD").InnerText Sej.ID = D.Item("ID").InnerText End Sub) End Sub)
Ok. So you're running a Parallel.For, and inside that, you're running a Parallel.ForEach. That's quite a lot of overhead just there, it's likely that you're shooting yourself in the foot here, I'd first remove the inner Parallel.ForEach:
Parallel.For(0, lst.Count, Sub(i) Dim P As XmlElement = GetElement(lst(i)) Dim ls = (From t In DocElet.ChildNodes Where t.item("ID").innerText = P.InnerText Select t) For Each foo In ls Dim verif_date As String = D.Item("DAD").InnerText Sej.ID = D.Item("ID").InnerText Next End Sub)
What's the purpose of verif_date? If it's not used, remove it:
For Each foo In ls Sej.ID = D.Item("ID").InnerText Next
Now the outer loop is making yet another selection:
Dim ls = (From t In DocElet.ChildNodes Where t.item("ID").innerText = P.InnerText Select t)
I'd try to flatten that. I'm not very familiar with the LINQ syntax in VB.NET, but what you want is a SelectMany, where you'd select the child nodes where "ID"'s inner text matches the element's inner text for the node you're iterating - in other words the ls query gets outside the outer loop and into the lst query, so you only need a single loop (which can probably run in parallel).
If the XML is massive, it could be worth trying some PLINQ:
Dim lst = (From t In DocElet.ChildNodes Select ID = t.item("ID").outerxml) .Distinct() .AsParallel() .SelectMany(...) .ToList()
Now you can iterate lst with Parallel.ForEach (time it with a normal For Each first) and do your thing.
As an aside, I have to say your naming is inconsistent and hard to follow: Sub(i) looks good (Sub(index) would be better), Sub(D) doesn't. I realize VB.NET isn't case-sensitive, but if you're going to have a convention to make function parameters camelCase, by all means stick to it. lst would probably be better off as elements or whatever makes sense - use real, readable words for your identifiers, not just arbitrary abbreviations.
