I still have the performance problem. I have attached the same project extende by a new button/(function to create a new group with one root entry and 100 sub-entries. This takes a while due to the log of the data context, but what really takes me off is the performance of the grid when opening this root entry. On my machine (i5, 4GB ram) it takes about 5-6 seconds for the sub-structure to open. The zip also contains an old version of the program and in the bin/debug folder you can find the "RadTest.exe" (as opposed to the RadTest2.exe), wich will open the sub-structure in no time.
So - there must be something wrong with my way of working the grid.

To reproduce: Run the program, click the "Generate" Button and enter a name for the new group. This will create 101 new entries (1 root, 100 subs), taking about 15-20 secs (which is ok, as I said above).
Then open the "RootEntry" within the group -> takes 5-6 seconds. When the subentries are visible, scrolling the grid vertically is really slow as well.