Why would invoking IHTMLElement2.getBoundingClientRect() from inside BHO get several times slower when switching from C++ to C#?

Question

I'm trying various options of creating a BHO for traversing HTML page DOM. One implementation uses C# and it's registered in registry with ApartmentModel set to Both. It goes like this:

1. retrieve IWebBrowser2.Document
2. obtain IDocumentSelector interface from document object
3. invoke IDocumentSelector.querySelectorAll("*") which yields a IHTMLDOMChildrenCollection reference
4. get IHTMLDOMChildrenCollection.length
5. run the for-loop in 0..length range (for(int index = 0; index < totalCount; index++)),
6. inside loop iteration obtain each collection item using IHTMLDOMChildrenCollection.item(),
7. cast the collection item reference to IHTMLElement2,
8. obtain IHTMLElement2.getClientBoundingRect()

and it works rather fine, a page with about 1500 elements gets traversed in 200-300 milliseconds (loop duration is measured by reading DateTime.UtcNow before and after the loop and getting TotalMilliseconds from the readings difference).

Another implementation in done with Visual C++ and ATL. It does mostly the same as the C# version. CComQIPtr is used in place of casts. The loop is the same. It's also registered with ApartmentModel set to Both.

The C++ implementation traverses the very same page DOM in 40-60 milliseconds. Time is measured by reading GetTickCount() before and after the loop and getting the difference.

Then I exclude the step 8 from inside loop iteration - item is obtained and IHTMLElement2 is obtained from it but getClientBoundingRect() is not invoked. After this change both implementations run in mostly the same time - 40-50 milliseconds.

This looks weird. Why would only getClientBoundingRect() be affected? What's so special in it that it slows down so much?